Move Singleton and related structs to namespace base

chrome/browser/extensions/install_tracker_factory.h

 // Copyright 2013 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_INSTALL_TRACKER_FACTORY_H_
 #define CHROME_BROWSER_EXTENSIONS_INSTALL_TRACKER_FACTORY_H_
 
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace extensions {
 
 class InstallTracker;
 
 class InstallTrackerFactory : public BrowserContextKeyedServiceFactory {
  public:
   static InstallTracker* GetForBrowserContext(content::BrowserContext* context);
   static InstallTrackerFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<InstallTrackerFactory>;
+  friend struct base::DefaultSingletonTraits<InstallTrackerFactory>;
 
   InstallTrackerFactory();
   ~InstallTrackerFactory() override;
 
   // BrowserContextKeyedServiceFactory overrides:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(InstallTrackerFactory);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_INSTALL_TRACKER_FACTORY_H_