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

components/offline_pages/core/prefetch/prefetch_service.h

Index: components/offline_pages/core/prefetch/prefetch_service.h
diff --git a/components/offline_pages/core/prefetch/prefetch_service.h b/components/offline_pages/core/prefetch/prefetch_service.h
index 057d9f03a9cb764d8a215a249df8d76af9d06949..f71ae79d798828a1a97f57c5331e7d0dc6557157 100644
--- a/components/offline_pages/core/prefetch/prefetch_service.h
+++ b/components/offline_pages/core/prefetch/prefetch_service.h
@@ -10,6 +10,10 @@
 
 class GURL;
 
+namespace gcm {
+class GCMDriver;
+}
+
 namespace offline_pages {
 
 // Main class for the Offline Pages Prefetching feature, which will ask a server
@@ -67,6 +71,10 @@ class PrefetchService : public KeyedService {
   // this call completes, the system will reschedule the task based on whether
   // SetNeedsReschedule has been called.
   virtual void StopBackgroundTask(ScopedBackgroundTask* task) = 0;
+
+  // When GCMDriver starts up, this method is called so that we can listen for
+  // messages corresponding to our app ID.
+  virtual void RegisterWithGCMDriver(gcm::GCMDriver* driver) = 0;
 };
 
 }  // namespace offline_pages