Migrate //chrome/browser/ui/webui/settings/ash to namespace ash [3/N]

This CL migrates os_settings_* files and included files (except
_sections*) to namespace ash.

This CL is part of the Chrome OS source code directory migration:
https://docs.google.com/document/d/1g-98HpzA8XcoGBWUv1gQNr4rbnD5yfvbtYZyPDDbkaE.

Bug: 1164001
Change-Id: I8d112840e02c4b2e8f39cec6ab8475a67635e3fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3937296
Owners-Override: Hidehiko Abe <[email protected]>
Reviewed-by: Hidehiko Abe <[email protected]>
Commit-Queue: Yeunjoo Choi <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1057415}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 207cac7..e8dacf7 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1494,7 +1494,7 @@
   chromeos::EduCoexistenceLoginHandler::RegisterProfilePrefs(registry);
   ash::SigninErrorNotifier::RegisterPrefs(registry);
   ash::ServicesCustomizationDocument::RegisterProfilePrefs(registry);
-  chromeos::settings::OSSettingsUI::RegisterProfilePrefs(registry);
+  ash::settings::OSSettingsUI::RegisterProfilePrefs(registry);
   ash::StartupUtils::RegisterOobeProfilePrefs(registry);
   ash::UserImageSyncObserver::RegisterProfilePrefs(registry);
   ChromeMetricsServiceClient::RegisterProfilePrefs(registry);