Move Singleton and related structs to namespace base

extensions/browser/extension_pref_value_map_factory.h

 // Copyright 2014 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 EXTENSIONS_BROWSER_EXTENSION_PREF_VALUE_MAP_FACTORY_H_
 #define EXTENSIONS_BROWSER_EXTENSION_PREF_VALUE_MAP_FACTORY_H_
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 class ExtensionPrefValueMap;
 
 // The usual factory boilerplate for ExtensionPrefValueMap.
 class ExtensionPrefValueMapFactory : public BrowserContextKeyedServiceFactory {
  public:
   static ExtensionPrefValueMap* GetForBrowserContext(
       content::BrowserContext* context);
 
   static ExtensionPrefValueMapFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<ExtensionPrefValueMapFactory>;
+  friend struct base::DefaultSingletonTraits<ExtensionPrefValueMapFactory>;
 
   ExtensionPrefValueMapFactory();
   ~ExtensionPrefValueMapFactory() override;
 
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
 };
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_PREF_VALUE_MAP_FACTORY_H_