Move Singleton and related structs to namespace base

chrome/browser/ui/app_list/search/history_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_UI_APP_LIST_SEARCH_HISTORY_FACTORY_H_
 #define CHROME_BROWSER_UI_APP_LIST_SEARCH_HISTORY_FACTORY_H_
 
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+namespace base {
 template<typename T> struct DefaultSingletonTraits;
+}
 
 namespace content {
 class BrowserContext;
 }
 
 namespace app_list {
 
 class History;
 
 // Singleton that owns all History and associates them with profiles;
 class HistoryFactory : public BrowserContextKeyedServiceFactory {
  public:
   // Returns singleton instance of HistoryFactory.
   static HistoryFactory* GetInstance();
 
   // Returns History associated with |context|.
   static History* GetForBrowserContext(content::BrowserContext* context);
 
  private:
-  friend struct DefaultSingletonTraits<HistoryFactory>;
+  friend struct base::DefaultSingletonTraits<HistoryFactory>;
 
   HistoryFactory();
   ~HistoryFactory() override;
 
   // BrowserContextKeyedServiceFactory overrides:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryFactory);
 };
 
 }  // namespace app_list
 
 #endif  // CHROME_BROWSER_UI_APP_LIST_SEARCH_HISTORY_FACTORY_H_