Move Singleton and related structs to namespace base

chrome/browser/notifications/extension_welcome_notification_factory.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_NOTIFICATIONS_EXTENSION_WELCOME_NOTIFICATION_FACTORY_H_
 #define CHROME_BROWSER_NOTIFICATIONS_EXTENSION_WELCOME_NOTIFICATION_FACTORY_H_
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 class ExtensionWelcomeNotification;
 
 // Singleton owning the extension welcome notification objects and associates
 // them with the browser context for which they may have to be shown.
 class ExtensionWelcomeNotificationFactory
     : public BrowserContextKeyedServiceFactory {
  public:
   // Returns the ExtensionWelcomeNotification instance to be used for |context|.
   static ExtensionWelcomeNotification* GetForBrowserContext(
       content::BrowserContext* context);
 
   static ExtensionWelcomeNotificationFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<ExtensionWelcomeNotificationFactory>;
+  friend struct base::DefaultSingletonTraits<
+      ExtensionWelcomeNotificationFactory>;
 
   ExtensionWelcomeNotificationFactory();
   ~ExtensionWelcomeNotificationFactory() override;
 
   // BrowserContextKeyedServiceFactory:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_EXTENSION_WELCOME_NOTIFICATION_FACTORY_H_