Migrate c/b/ash/login/easy_unlock/ to namespace ash

The migration of chromeos/browser/chromeos/login/easy_unlock/ was
finished in https://crrev.com/c/2721996. This CL changes the namespace
from chromeos to ash. Compatibility code is added until the Chrome OS
source code migration is finished.

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

Bug: 1164001
Change-Id: Iee4622152d73082cf909415527194e7d84290bc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2883742
Reviewed-by: Hidehiko Abe <[email protected]>
Owners-Override: Hidehiko Abe <[email protected]>
Commit-Queue: Henrique Ferreiro <[email protected]>
Cr-Commit-Position: refs/heads/master@{#881556}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 27d2a968..fc022a0 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -807,7 +807,7 @@
   chromeos::DeviceNameStore::RegisterLocalStatePrefs(registry);
   chromeos::DeviceOAuth2TokenStoreChromeOS::RegisterPrefs(registry);
   ash::device_settings_cache::RegisterPrefs(registry);
-  chromeos::EasyUnlockService::RegisterPrefs(registry);
+  ash::EasyUnlockService::RegisterPrefs(registry);
   chromeos::echo_offer::RegisterPrefs(registry);
   chromeos::EnableAdbSideloadingScreen::RegisterPrefs(registry);
   chromeos::EnableDebuggingScreenHandler::RegisterPrefs(registry);