Move Singleton and related structs to namespace base

extensions/browser/event_router_factory.h

 // Copyright 2015 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 EXTENSIONS_BROWSER_EVENT_ROUTER_FACTORY_H_
 #define EXTENSIONS_BROWSER_EVENT_ROUTER_FACTORY_H_
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 namespace extensions {
 
 class EventRouter;
 
 class EventRouterFactory : public BrowserContextKeyedServiceFactory {
  public:
   static EventRouter* GetForBrowserContext(content::BrowserContext* context);
   static EventRouterFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<EventRouterFactory>;
+  friend struct base::DefaultSingletonTraits<EventRouterFactory>;
 
   EventRouterFactory();
   ~EventRouterFactory() override;
 
   // BrowserContextKeyedServiceFactory implementation
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(EventRouterFactory);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EVENT_ROUTER_FACTORY_H_