Change namespace chromeos => ash for //c/b/ash/system

Code in //chrome/browser/ash/system 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: Iec510bb637485ce5a2440849df2f7ec64dc5c5cc
Bug: 1164001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2703815
Reviewed-by: James Cook <[email protected]>
Owners-Override: James Cook <[email protected]>
Commit-Queue: Yeunjoo Choi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#855445}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index cbe7bd9..82a9a90 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -798,7 +798,7 @@
   chromeos::SigninScreenHandler::RegisterPrefs(registry);
   chromeos::StartupUtils::RegisterPrefs(registry);
   chromeos::StatsReportingController::RegisterLocalStatePrefs(registry);
-  chromeos::system::AutomaticRebootManager::RegisterPrefs(registry);
+  ash::system::AutomaticRebootManager::RegisterPrefs(registry);
   chromeos::TimeZoneResolver::RegisterPrefs(registry);
   chromeos::UserImageManager::RegisterPrefs(registry);
   chromeos::UserSessionManager::RegisterPrefs(registry);