Move Singleton and related structs to namespace base

extensions/shell/browser/shell_extension_system_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_SHELL_BROWSER_SHELL_EXTENSION_SYSTEM_FACTORY_H_
 #define EXTENSIONS_SHELL_BROWSER_SHELL_EXTENSION_SYSTEM_FACTORY_H_
 
 #include "base/memory/singleton.h"
 #include "extensions/browser/extension_system_provider.h"
 
 namespace extensions {
 
 // A factory that provides ShellExtensionSystem for app_shell.
 class ShellExtensionSystemFactory : public ExtensionSystemProvider {
  public:
   // ExtensionSystemProvider implementation:
   ExtensionSystem* GetForBrowserContext(
       content::BrowserContext* context) override;
 
   static ShellExtensionSystemFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<ShellExtensionSystemFactory>;
+  friend struct base::DefaultSingletonTraits<ShellExtensionSystemFactory>;
 
   ShellExtensionSystemFactory();
   ~ShellExtensionSystemFactory() override;
 
   // BrowserContextKeyedServiceFactory implementation:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
   bool ServiceIsCreatedWithBrowserContext() const override;
 
   DISALLOW_COPY_AND_ASSIGN(ShellExtensionSystemFactory);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_SHELL_BROWSER_SHELL_EXTENSION_SYSTEM_FACTORY_H_