Move Singleton and related structs to namespace base

chrome/browser/extensions/extension_storage_monitor_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 CHROME_BROWSER_EXTENSIONS_EXTENSION_STORAGE_MONITOR_FACTORY_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_STORAGE_MONITOR_FACTORY_H_
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 namespace extensions {
 
 class ExtensionStorageMonitor;
 
 class ExtensionStorageMonitorFactory
     : public BrowserContextKeyedServiceFactory {
  public:
   static ExtensionStorageMonitor* GetForBrowserContext(
       content::BrowserContext* context);
 
   static ExtensionStorageMonitorFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<ExtensionStorageMonitorFactory>;
+  friend struct base::DefaultSingletonTraits<ExtensionStorageMonitorFactory>;
 
   ExtensionStorageMonitorFactory();
   ~ExtensionStorageMonitorFactory() override;
 
   // BrowserContextKeyedServiceFactory implementation:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
   bool ServiceIsCreatedWithBrowserContext() const override;
   bool ServiceIsNULLWhileTesting() const override;
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_STORAGE_MONITOR_FACTORY_H_