Move Singleton and related structs to namespace base

chrome/browser/android/offline_pages/offline_page_model_factory.h

 // Copyright 2015 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_ANDROID_OFFLINE_PAGES_OFFLINE_PAGE_MODEL_FACTORY_H_
 #define CHROME_BROWSER_ANDROID_OFFLINE_PAGES_OFFLINE_PAGE_MODEL_FACTORY_H_
 
 #include "base/macros.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}  // namespace base
 
 namespace offline_pages {
 
 class OfflinePageModel;
 
 // A factory to create one unique OfflinePageModel.
 class OfflinePageModelFactory : public BrowserContextKeyedServiceFactory {
  public:
   static OfflinePageModelFactory* GetInstance();
   static OfflinePageModel* GetForBrowserContext(
       content::BrowserContext* context);
 
  private:
-  friend struct DefaultSingletonTraits<OfflinePageModelFactory>;
+  friend struct base::DefaultSingletonTraits<OfflinePageModelFactory>;
 
   OfflinePageModelFactory();
   ~OfflinePageModelFactory() override {}
 
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
 
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(OfflinePageModelFactory);
 };
 
 }  // namespace offline_pages
 
 #endif  // CHROME_BROWSER_ANDROID_OFFLINE_PAGES_OFFLINE_PAGE_MODEL_FACTORY_H_