一、先说结论 User Data 路径优先级如下：

1、注册表中策略配置的路径。

   计算机\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Chromium

    UserDataDir="xx_path"

2、命令行中的路径。

   --user-data-dir="xx_path"

3、默认用户路径

   %LOCALAPPDATA%/Chromium/User Data

二、接下来上源码分析过程

一、chrome\app\chrome_crash_reporter_client_win.cc

    //1、开始初始化user data目录
    std::wstring user_data_dir;
    if (process_type.empty())
      install_static::GetUserDataDirectory(&user_data_dir, nullptr);

// static
void ChromeCrashReporterClient::InitializeCrashReportingForProcess() {
  static ChromeCrashReporterClient* instance = nullptr;
  if (instance)
    return;

  instance = new ChromeCrashReporterClient();
  ANNOTATE_LEAKING_OBJECT_PTR(instance);

  std::wstring process_type = install_static::GetSwitchValueFromCommandLine(
      ::GetCommandLine(), install_static::kProcessType);
  // Don't set up Crashpad crash reporting in the Crashpad handler itself, nor
  // in the fallback crash handler for the Crashpad handler process.
  if (process_type != install_static::kCrashpadHandler &&
      process_type != install_static::kFallbackHandler) {
    crash_reporter::SetCrashReporterClient(instance);
    
    //1、开始初始化user data目录
    std::wstring user_data_dir;
    if (process_type.empty())
      install_static::GetUserDataDirectory(&user_data_dir, nullptr);

    // TODO(wfh): Add a DCHECK for success. See https://crbug.com/1329269.
    std::ignore = crash_reporter::InitializeCrashpadWithEmbeddedHandler(
        /*initial_client=*/process_type.empty(),
        install_static::WideToUTF8(process_type),
        install_static::WideToUTF8(user_data_dir), base::FilePath());
  }
}

二、第二步看install_static::GetUserDataDirectory(&user_data_dir, nullptr);实现

 在chrome\install_static\user_data_dir.cc中

namespace install_static {

namespace {

std::wstring* g_user_data_dir;
std::wstring* g_invalid_user_data_dir;

// Retrieves a registry policy for the user data directory from the registry, if
// one is set. If there's none set in either HKLM or HKCU, |user_data_dir| will
// be unmodified.
void GetUserDataDirFromRegistryPolicyIfSet(const InstallConstants& mode,
                                           std::wstring* user_data_dir) {
  assert(user_data_dir);
  std::wstring policies_path = L"SOFTWARE\\Policies\\";
  AppendChromeInstallSubDirectory(mode, false /* !include_suffix */,
                                  &policies_path);

  std::wstring value;

  constexpr wchar_t kUserDataDirRegistryKeyName[] = L"UserDataDir";

  // First, try HKLM.
  if (nt::QueryRegValueSZ(nt::HKLM, nt::NONE, policies_path.c_str(),
                          kUserDataDirRegistryKeyName, &value)) {
    *user_data_dir = ExpandPathVariables(value);
    return;
  }

  // Second, try HKCU.
  if (nt::QueryRegValueSZ(nt::HKCU, nt::NONE, policies_path.c_str(),
                          kUserDataDirRegistryKeyName, &value)) {
    *user_data_dir = ExpandPathVariables(value);
    return;
  }
}

std::wstring MakeAbsoluteFilePath(const std::wstring& input) {
  wchar_t file_path[MAX_PATH];
  if (!_wfullpath(file_path, input.c_str(), _countof(file_path)))
    return std::wstring();
  return file_path;
}

// The same as GetUserDataDirectory(), but directly queries the global command
// line object for the --user-data-dir flag. This is the more commonly used
// function, where GetUserDataDirectory() is used primiarily for testing.
bool GetUserDataDirectoryUsingProcessCommandLine(
    const InstallConstants& mode,
    std::wstring* result,
    std::wstring* invalid_supplied_directory) {
  return GetUserDataDirectoryImpl(
      GetSwitchValueFromCommandLine(::GetCommandLine(), kUserDataDirSwitch),
      mode, result, invalid_supplied_directory);
}

// Populates |result| with the default User Data directory for the current
// user. Returns false if all attempts at locating a User Data directory fail.
// TODO(ananta)
// http://crbug.com/604923
// Unify this with the Browser Distribution code.
bool GetDefaultUserDataDirectory(const InstallConstants& mode,
                                 std::wstring* result) {
  // This environment variable should be set on Windows Vista and later
  // (https://msdn.microsoft.com/library/windows/desktop/dd378457.aspx).
  std::wstring user_data_dir = GetEnvironmentString(L"LOCALAPPDATA");

  if (user_data_dir.empty()) {
    // LOCALAPPDATA was not set; fallback to the temporary files path.
    DWORD size = ::GetTempPath(0, nullptr);
    if (!size)
      return false;
    user_data_dir.resize(size + 1);
    size = ::GetTempPath(size + 1, &user_data_dir[0]);
    if (!size || size >= user_data_dir.size())
      return false;
    user_data_dir.resize(size);
  }

  result->swap(user_data_dir);
  if ((*result)[result->length() - 1] != L'\\')
    result->push_back(L'\\');
  AppendChromeInstallSubDirectory(mode, true /* include_suffix */, result);
  result->push_back(L'\\');
  result->append(L"User Data");
  return true;
}

}  // namespace

bool GetUserDataDirectoryImpl(
    const std::wstring& user_data_dir_from_command_line,
    const InstallConstants& mode,
    std::wstring* result,
    std::wstring* invalid_supplied_directory) {
  std::wstring user_data_dir = user_data_dir_from_command_line;
  
  //从注册表中读取路径
  GetUserDataDirFromRegistryPolicyIfSet(mode, &user_data_dir);

  // On Windows, trailing separators leave Chrome in a bad state. See
  // crbug.com/464616.
  while (!user_data_dir.empty() &&
         (user_data_dir.back() == '\\' || user_data_dir.back() == '/')) {
    user_data_dir.pop_back();
  }

  bool got_valid_directory =
      !user_data_dir.empty() && RecursiveDirectoryCreate(user_data_dir);
  if (!got_valid_directory) {
    *invalid_supplied_directory = user_data_dir;
    //从%LOCALAPPDATA%/Chromium/User Data 中读取
    got_valid_directory = GetDefaultUserDataDirectory(mode, &user_data_dir);
  }

  // The Chrome implementation CHECKs() here in the browser process. We
  // don't as this function is used to initialize crash reporting, so
  // we would get no report of this failure.
  assert(got_valid_directory);
  if (!got_valid_directory)
    return false;

  *result = MakeAbsoluteFilePath(user_data_dir);
  return true;
}

bool GetUserDataDirectory(std::wstring* user_data_dir,
                          std::wstring* invalid_user_data_dir) {
  if (!g_user_data_dir) {
    g_user_data_dir = new std::wstring();
    g_invalid_user_data_dir = new std::wstring();
    //从命令行中解析路径
    if (!GetUserDataDirectoryUsingProcessCommandLine(
            InstallDetails::Get().mode(), g_user_data_dir,
            g_invalid_user_data_dir)) {
      return false;
    }
    assert(!g_user_data_dir->empty());
  }
  *user_data_dir = *g_user_data_dir;
  if (invalid_user_data_dir)
    *invalid_user_data_dir = *g_invalid_user_data_dir;
  return true;
}

}  // namespace install_static

调用堆栈:

GetUserDataDirectory->GetUserDataDirectoryUsingProcessCommandLine->

GetUserDataDirectoryImpl->GetSwitchValueFromCommandLine->GetUserDataDirFromRegistryPolicyIfSet->GetDefaultUserDataDirectory

   

三、最后将初始化的g_user_data_dir路径设置到   

chrome\app\chrome_main_delegate.cc

CHECK(base::PathService::OverrideAndCreateIfNeeded(
        chrome::DIR_USER_DATA, user_data_dir, false, true));中，

// Initializes the user data dir. Must be called before InitializeLocalState().
void InitializeUserDataDir(base::CommandLine* command_line) {
#if BUILDFLAG(IS_CHROMEOS_LACROS)
  // In debug builds of Lacros, we keep track of when the user data dir
  // is initialized, to ensure the cryptohome is not accessed before login
  // when prelaunching at login screen.
  chromeos::lacros_paths::SetInitializedUserDataDir();
#endif
#if BUILDFLAG(IS_WIN)
  // Reach out to chrome_elf for the truth on the user data directory.
  // Note that in tests, this links to chrome_elf_test_stubs.
  wchar_t user_data_dir_buf[MAX_PATH], invalid_user_data_dir_buf[MAX_PATH];

  // In tests this may return false, implying the user data dir should be unset.
  if (GetUserDataDirectoryThunk(user_data_dir_buf, std::size(user_data_dir_buf),
                                invalid_user_data_dir_buf,
                                std::size(invalid_user_data_dir_buf))) {
    base::FilePath user_data_dir(user_data_dir_buf);
    if (invalid_user_data_dir_buf[0] != 0) {
      chrome::SetInvalidSpecifiedUserDataDir(
          base::FilePath(invalid_user_data_dir_buf));
      command_line->AppendSwitchPath(switches::kUserDataDir, user_data_dir);
    }
    //将user data路径存储到path缓存中
    CHECK(base::PathService::OverrideAndCreateIfNeeded(
        chrome::DIR_USER_DATA, user_data_dir, false, true));
  }

这样

  base::FilePath user_data_dir;

  base::PathService::Get(chrome::DIR_USER_DATA, &user_data_dir);

就可以读取其中的路径了。