Move Singleton and related structs to namespace base

ios/chrome/browser/signin/account_tracker_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_SIGNIN_ACCOUNT_TRACKER_SERVICE_FACTORY_H_
 #define IOS_CHROME_BROWSER_SIGNIN_ACCOUNT_TRACKER_SERVICE_FACTORY_H_
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
 
 class AccountTrackerService;
+
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}  // namespace base
 
 namespace ios {
 
 class ChromeBrowserState;
 
 // Singleton that owns all AccountTrackerServices and associates them with
 // ios::ChromeBrowserState.
 class AccountTrackerServiceFactory : public BrowserStateKeyedServiceFactory {
  public:
   // Returns the instance of AccountTrackerService associated with this
   // browser state (creating one if none exists). Returns nullptr if this
   // browser state cannot have a AccountTrackerService (for example, if
   // |browser_state| is incognito).
   static AccountTrackerService* GetForBrowserState(
       ios::ChromeBrowserState* browser_state);
 
   // Returns the instance of the AccountTrackerServiceFactory singleton.
   static AccountTrackerServiceFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<AccountTrackerServiceFactory>;
+  friend struct base::DefaultSingletonTraits<AccountTrackerServiceFactory>;
 
   AccountTrackerServiceFactory();
   ~AccountTrackerServiceFactory() override;
 
   // BrowserStateKeyedServiceFactory implementation.
   void RegisterBrowserStatePrefs(
       user_prefs::PrefRegistrySyncable* registry) override;
   scoped_ptr<KeyedService> BuildServiceInstanceFor(
       web::BrowserState* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(AccountTrackerServiceFactory);
 };
 
 }  // namespace ios
 
 #endif  // IOS_CHROME_BROWSER_SIGNIN_ACCOUNT_TRACKER_SERVICE_FACTORY_H_