Move Singleton and related structs to namespace base

ios/chrome/browser/history/top_sites_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_TOP_SITES_FACTORY_H_
 #define IOS_CHROME_BROWSER_HISTORY_TOP_SITES_FACTORY_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "components/keyed_service/ios/refcounted_browser_state_keyed_service_factory.h"
-
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}  // namespace base
 
 namespace history {
 class TopSites;
 }
 
 namespace ios {
 
 class ChromeBrowserState;
 
 // TopSitesFactory is a singleton that associates history::TopSites instance to
 // ChromeBrowserState.
 class TopSitesFactory : public RefcountedBrowserStateKeyedServiceFactory {
  public:
   static scoped_refptr<history::TopSites> GetForBrowserState(
       ios::ChromeBrowserState* browser_state);
   static TopSitesFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<TopSitesFactory>;
+  friend struct base::DefaultSingletonTraits<TopSitesFactory>;
 
   TopSitesFactory();
   ~TopSitesFactory() override;
 
   // RefcountedBrowserStateKeyedServiceFactory implementation.
   scoped_refptr<RefcountedKeyedService> BuildServiceInstanceFor(
       web::BrowserState* context) const override;
   void RegisterBrowserStatePrefs(
       user_prefs::PrefRegistrySyncable* registry) override;
   bool ServiceIsNULLWhileTesting() const override;
 
   DISALLOW_COPY_AND_ASSIGN(TopSitesFactory);
 };
 
 }  // namespace ios
 
 #endif  // IOS_CHROME_BROWSER_HISTORY_TOP_SITES_FACTORY_H_