Abstract history dependencies on bookmarks through HistoryClient

chrome/browser/history/chrome_history_client_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 CHROME_BROWSER_HISTORY_CHROME_HISTORY_CLIENT_FACTORY_H_
 #define CHROME_BROWSER_HISTORY_CHROME_HISTORY_CLIENT_FACTORY_H_
 
-#include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 class ChromeHistoryClient;
 class Profile;
 
 // Singleton that owns all ChromeHistoryClients and associates them with
 // Profiles.
 class ChromeHistoryClientFactory : public BrowserContextKeyedServiceFactory {
  public:
   static ChromeHistoryClient* GetForProfile(Profile* profile);
 
   static ChromeHistoryClientFactory* GetInstance();
 
  private:
   friend struct DefaultSingletonTraits<ChromeHistoryClientFactory>;
 
   ChromeHistoryClientFactory();
   virtual ~ChromeHistoryClientFactory();
 
   // BrowserContextKeyedServiceFactory:
   virtual KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const OVERRIDE;
   virtual content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const OVERRIDE;
   virtual bool ServiceIsNULLWhileTesting() const OVERRIDE;
 };
 
 #endif  // CHROME_BROWSER_HISTORY_CHROME_HISTORY_CLIENT_FACTORY_H_