Move Singleton and related structs to namespace base

chrome/browser/undo/bookmark_undo_service_factory.h

 // Copyright 2013 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_UNDO_BOOKMARK_UNDO_SERVICE_FACTORY_H_
 #define CHROME_BROWSER_UNDO_BOOKMARK_UNDO_SERVICE_FACTORY_H_
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 class BookmarkUndoService;
 class Profile;
 
 class BookmarkUndoServiceFactory : public BrowserContextKeyedServiceFactory {
  public:
   static BookmarkUndoService* GetForProfile(Profile* profile);
 
   static BookmarkUndoService* GetForProfileIfExists(Profile* profile);
 
   static BookmarkUndoServiceFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<BookmarkUndoServiceFactory>;
+  friend struct base::DefaultSingletonTraits<BookmarkUndoServiceFactory>;
 
   BookmarkUndoServiceFactory();
   ~BookmarkUndoServiceFactory() override;
 
   // BrowserContextKeyedServiceFactory:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(BookmarkUndoServiceFactory);
 };
 
 #endif  // CHROME_BROWSER_UNDO_BOOKMARK_UNDO_SERVICE_FACTORY_H_