Make PrefService available in *ash

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 9bc373d2203cccc5bd689a37ffd133162aae4285..7e42d78b5736f92d52b78a8e79c452fd5a8d4f2e 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -376,6 +376,13 @@ void Shell::UpdateShelfVisibility() {
     root->GetRootWindowController()->GetShelf()->UpdateVisibilityState();
 }
 
+PrefService* Shell::GetActiveUserPrefService() const {
+  if (shell_port_->GetAshConfig() == Config::MASH)
+    return pref_service_.get();
+
+  return shell_delegate_->GetActiveUserPrefService();
+}
+
 WebNotificationTray* Shell::GetWebNotificationTray() {
   return GetPrimaryRootWindowController()
       ->GetStatusAreaWidget()