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

components/offline_pages/core/prefetch/prefetch_service.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_CORE_PREFETCH_PREFETCH_SERVICE_H_
 #define COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_SERVICE_H_
 
 #include "components/keyed_service/core/keyed_service.h"
 
+namespace ntp_snippets {
+class ContentSuggestionsService;
+}
+
 namespace offline_pages {
 
 class PrefetchDispatcher;
+class PrefetchGCMHandler;
 
 // Main class and entry point for the Offline Pages Prefetching feature, that
 // controls the lifetime of all major subcomponents of the prefetching system.
 class PrefetchService : public KeyedService {
  public:
   ~PrefetchService() override = default;
 
   virtual PrefetchDispatcher* GetDispatcher() = 0;
+  virtual PrefetchGCMHandler* GetPrefetchGCMHandler() = 0;
+
+  // Called at construction of the ContentSuggestionsService to begin observing
+  // events related to incoming articles.
+  virtual void ObserveContentSuggestionsService(
+      ntp_snippets::ContentSuggestionsService* service) = 0;
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_SERVICE_H_