Move Singleton and related structs to namespace base

extensions/browser/api/vpn_provider/vpn_service_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_API_VPN_PROVIDER_VPN_SERVICE_FACTORY_H_
 #define EXTENSIONS_BROWSER_API_VPN_PROVIDER_VPN_SERVICE_FACTORY_H_
 
 #include "base/macros.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 namespace content {
 
 class BrowserContext;
 
 }  // namespace content
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}
 
 namespace chromeos {
 
 class VpnService;
 
 // Factory to create VpnService.
 class VpnServiceFactory : public BrowserContextKeyedServiceFactory {
  public:
   static VpnService* GetForBrowserContext(content::BrowserContext* context);
   static VpnServiceFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<VpnServiceFactory>;
+  friend struct base::DefaultSingletonTraits<VpnServiceFactory>;
 
   VpnServiceFactory();
   ~VpnServiceFactory() override;
 
   // BrowserContextKeyedServiceFactory:
   bool ServiceIsCreatedWithBrowserContext() const override;
   bool ServiceIsNULLWhileTesting() const override;
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(VpnServiceFactory);
 };
 
 }  // namespace chromeos
 
 #endif  // EXTENSIONS_BROWSER_API_VPN_PROVIDER_VPN_SERVICE_FACTORY_H_