Move Singleton and related structs to namespace base

chrome/browser/autocomplete/in_memory_url_index_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_AUTOCOMPLETE_IN_MEMORY_URL_INDEX_FACTORY_H_
 #define CHROME_BROWSER_AUTOCOMPLETE_IN_MEMORY_URL_INDEX_FACTORY_H_
 
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 class InMemoryURLIndex;
 class Profile;
 
 class InMemoryURLIndexFactory : public BrowserContextKeyedServiceFactory {
  public:
   static InMemoryURLIndex* GetForProfile(Profile* profile);
   static InMemoryURLIndexFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<InMemoryURLIndexFactory>;
+  friend struct base::DefaultSingletonTraits<InMemoryURLIndexFactory>;
 
   InMemoryURLIndexFactory();
   ~InMemoryURLIndexFactory() override;
 
   // BrowserContextKeyedServiceFactory:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
   bool ServiceIsNULLWhileTesting() const override;
 };
 
 #endif  // CHROME_BROWSER_AUTOCOMPLETE_IN_MEMORY_URL_INDEX_FACTORY_H_