Move Singleton and related structs to namespace base

chrome/browser/bookmarks/bookmark_model_factory.h

 // Copyright (c) 2012 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 CHROME_BROWSER_BOOKMARKS_BOOKMARK_MODEL_FACTORY_H_
 #define CHROME_BROWSER_BOOKMARKS_BOOKMARK_MODEL_FACTORY_H_
 
 #include "base/macros.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 class Profile;
 
 namespace bookmarks {
 class BookmarkModel;
 }
 
 // Singleton that owns all BookmarkModels and associates them with Profiles.
 class BookmarkModelFactory : public BrowserContextKeyedServiceFactory {
  public:
   static bookmarks::BookmarkModel* GetForProfile(Profile* profile);
 
   static bookmarks::BookmarkModel* GetForProfileIfExists(Profile* profile);
 
   static BookmarkModelFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<BookmarkModelFactory>;
+  friend struct base::DefaultSingletonTraits<BookmarkModelFactory>;
 
   BookmarkModelFactory();
   ~BookmarkModelFactory() override;
 
   // BrowserContextKeyedServiceFactory:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   void RegisterProfilePrefs(
       user_prefs::PrefRegistrySyncable* registry) override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
   bool ServiceIsNULLWhileTesting() const override;
 
   DISALLOW_COPY_AND_ASSIGN(BookmarkModelFactory);
 };
 
 #endif  // CHROME_BROWSER_BOOKMARKS_BOOKMARK_MODEL_FACTORY_H_