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

Code in //chrome/browser/ash/first_run 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: Ib006d248241552c8d21860f75d5d9fdc8c5084ae
Bug: 1164001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3041051
Reviewed-by: Hidehiko Abe <[email protected]>
Owners-Override: Hidehiko Abe <[email protected]>
Commit-Queue: Yeunjoo Choi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#904141}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 2f10fe8..8c156eb 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1268,7 +1268,7 @@
   ash::FamilyUserMetricsService::RegisterProfilePrefs(registry);
   ash::FamilyUserSessionMetrics::RegisterProfilePrefs(registry);
   chromeos::InlineLoginHandlerChromeOS::RegisterProfilePrefs(registry);
-  chromeos::first_run::RegisterProfilePrefs(registry);
+  ash::first_run::RegisterProfilePrefs(registry);
   ash::file_system_provider::RegisterProfilePrefs(registry);
   chromeos::full_restore::RegisterProfilePrefs(registry);
   ash::KerberosCredentialsManager::RegisterProfilePrefs(registry);