Introduce HistoryClient interface

chrome/browser/history/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_HISTORY_CLIENT_FACTORY_H_
+#define CHROME_BROWSER_HISTORY_HISTORY_CLIENT_FACTORY_H_
+
+#include "base/memory/singleton.h"
+#include "components/keyed_service/content/browser_context_keyed_service_factory.h"
+
+class Profile;
+
+namespace history {
+class HistoryClient;
+}
+
+// Singleton that owns all HistoryClient and associates them with Profiles.
+class HistoryClientFactory : public BrowserContextKeyedServiceFactory {
+ public:
+  static history::HistoryClient* GetForProfile(Profile* profile);
+
+  static history::HistoryClient* GetForProfileIfExists(Profile* profile);
+
+  static HistoryClientFactory* GetInstance();
+
+ private:
+  friend struct DefaultSingletonTraits<HistoryClientFactory>;
+
+  HistoryClientFactory();
+  virtual ~HistoryClientFactory();
+
+  // 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_HISTORY_CLIENT_FACTORY_H_