[Offline Pages] Add a GCMAppHandler for offline page prefetch.

components/offline_pages/content/prefetch_service_factory.h

-// Copyright 2017 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 COMPONENTS_OFFLINE_PAGES_CONTENT_PREFETCH_SERVICE_FACTORY_H_
-#define COMPONENTS_OFFLINE_PAGES_CONTENT_PREFETCH_SERVICE_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 PrefetchService;
-
-// A factory to create one PrefetchServiceImpl per browser context. Prefetching
-// Offline Pages is not supported in incognito, so this class uses default
-// implementation of |GetBrowserContextToUse|.
-class PrefetchServiceFactory : public BrowserContextKeyedServiceFactory {
- public:
-  static PrefetchServiceFactory* GetInstance();
-  static PrefetchService* GetForBrowserContext(
-      content::BrowserContext* context);
-
- private:
-  friend struct base::DefaultSingletonTraits<PrefetchServiceFactory>;
-
-  PrefetchServiceFactory();
-  ~PrefetchServiceFactory() override {}
-
-  KeyedService* BuildServiceInstanceFor(
-      content::BrowserContext* context) const override;
-
-  DISALLOW_COPY_AND_ASSIGN(PrefetchServiceFactory);
-};
-
-}  // namespace offline_pages
-
-#endif  // COMPONENTS_OFFLINE_PAGES_CONTENT_PREFETCH_SERVICE_FACTORY_H_