Move Singleton and related structs to namespace base

ios/chrome/browser/history/web_history_service_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 IOS_CHROME_BROWSER_HISTORY_WEB_HISTORY_SERVICE_FACTORY_H_
 #define IOS_CHROME_BROWSER_HISTORY_WEB_HISTORY_SERVICE_FACTORY_H_
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
 
 template <typename T>
-struct DefaultSingletonTraits;
+struct base::DefaultSingletonTraits;
 
 namespace history {
 class WebHistoryService;
 }
 
 namespace ios {
 
 class ChromeBrowserState;
 
 // Singleton that owns all WebHistoryServices and associates them with
 // ios::ChromeBrowserState.
 class WebHistoryServiceFactory : public BrowserStateKeyedServiceFactory {
  public:
   static history::WebHistoryService* GetForBrowserState(
       ios::ChromeBrowserState* browser_state);
   static WebHistoryServiceFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<WebHistoryServiceFactory>;
+  friend struct base::DefaultSingletonTraits<WebHistoryServiceFactory>;
 
   WebHistoryServiceFactory();
   ~WebHistoryServiceFactory() override;
 
   // BrowserStateKeyedServiceFactory implementation.
   scoped_ptr<KeyedService> BuildServiceInstanceFor(
       web::BrowserState* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(WebHistoryServiceFactory);
 };
 
 }  // namespace ios
 
 #endif  // IOS_CHROME_BROWSER_HISTORY_WEB_HISTORY_SERVICE_FACTORY_H_