Change namespace to ash for //c/b/ash/file_system_provider/* files

Code in //chrome/browser/ash/file_system_provider/* files is a part
of the ash-chrome, so namespace is changed to ash from chromeos.

To avoid churn during other namespace transitions in future, this CL has
temporary 'using' declarations.

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

Change-Id: Id0f9d9846429d117bab813f1fdf44f820253f690
Bug: 1164001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2896752
Reviewed-by: Hidehiko Abe <[email protected]>
Owners-Override: Hidehiko Abe <[email protected]>
Commit-Queue: Yeunjoo Choi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#890561}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 26723c3..6946d30 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1151,7 +1151,7 @@
   ash::FamilyUserSessionMetrics::RegisterProfilePrefs(registry);
   chromeos::InlineLoginHandlerChromeOS::RegisterProfilePrefs(registry);
   chromeos::first_run::RegisterProfilePrefs(registry);
-  chromeos::file_system_provider::RegisterProfilePrefs(registry);
+  ash::file_system_provider::RegisterProfilePrefs(registry);
   chromeos::full_restore::RegisterProfilePrefs(registry);
   ash::KerberosCredentialsManager::RegisterProfilePrefs(registry);
   chromeos::login::SecurityTokenSessionController::RegisterProfilePrefs(