Move Singleton and related structs to namespace base

ios/chrome/browser/history/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_HISTORY_SERVICE_FACTORY_H_
 #define IOS_CHROME_BROWSER_HISTORY_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"
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}  // namespace base
+
 enum class ServiceAccessType;
 
 namespace history {
 class HistoryService;
 }
 
 namespace ios {
 
 class ChromeBrowserState;
 
 // Singleton that owns all HistoryServices and associates them with
 // ios::ChromeBrowserState.
 class HistoryServiceFactory : public BrowserStateKeyedServiceFactory {
  public:
   static history::HistoryService* GetForBrowserState(
       ios::ChromeBrowserState* browser_state,
       ServiceAccessType access_type);
   static history::HistoryService* GetForBrowserStateIfExists(
       ios::ChromeBrowserState* browser_state,
       ServiceAccessType access_type);
   static HistoryServiceFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<HistoryServiceFactory>;
+  friend struct base::DefaultSingletonTraits<HistoryServiceFactory>;
 
   HistoryServiceFactory();
   ~HistoryServiceFactory() override;
 
   // BrowserStateKeyedServiceFactory implementation.
   scoped_ptr<KeyedService> BuildServiceInstanceFor(
       web::BrowserState* context) const override;
   web::BrowserState* GetBrowserStateToUse(
       web::BrowserState* context) const override;
   bool ServiceIsNULLWhileTesting() const override;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryServiceFactory);
 };
 
 }  // namespace ios
 
 #endif  // IOS_CHROME_BROWSER_HISTORY_HISTORY_SERVICE_FACTORY_H_