Fix OnRegisterCustomPreferences timing and add SetDefaultPreference

[magreenblatt]

CefBrowserProcessHandler::OnRegisterCustomPreferences for CEF_PREFERENCES_TYPE_REQUEST_CONTEXT is currently called after Chromium's general profile preferences are registered in RegisterUserProfilePrefs, but before service-specific preferences are registered in RegisterProfilePrefs. This timing is incorrect for values like prefs::kSigninAllowedOnNextStartup, which can only be configured after the service-specific registration is complete (see issue #4276). The current timing also introduces the possibility of CEF apps registering preference names that conflict with Chromium service-specific preferences.

The proposal here is to move OnRegisterCustomPreferences timing after all Chromium preferences are registered (end of RegisterProfilePrefs in pref_service_builder_utils.cc), and add a new CefPreferenceRegistrar::SetDefaultPreference method that can be used to override defaults for all Chromium preferences. Persisted user settings and higher-priority settings (set via CefPreferenceManager::SetPreference, managed settings or command-line) would still take precedence over these default values (standard Chromium behavior for defaults).

In summary:

• Remove the CEF registration call from Chromium’s RegisterUserProfilePrefs().
• Add it at the end of RegisterProfilePrefs() in pref_service_builder_utils.cc (chrome/browser/profiles/pref_service_builder_utils.cc:66), after both dependency managers register their preferences.
• Add a new CefPreferenceRegistrar::SetDefaultPreference method that calls PrefRegistry::SetDefaultPrefValue()

  virtual bool SetDefaultPreference(const CefString& name,
                                   CefRefPtr<CefValue> value,
                                   CefString& error) = 0;