Push API: register PushMessagingServiceImpl as GCMAppHandler.

components/gcm_driver/gcm_app_handler.h

Index: components/gcm_driver/gcm_app_handler.h
diff --git a/components/gcm_driver/gcm_app_handler.h b/components/gcm_driver/gcm_app_handler.h
index b4b497e3975fa4fa5fb408f126b4746c8b760025..d6d4bc3bf449bb403bb74a053be073f24dc8cb2a 100644
--- a/components/gcm_driver/gcm_app_handler.h
+++ b/components/gcm_driver/gcm_app_handler.h
@@ -45,6 +45,11 @@ class GCMAppHandler {
   // Called when the connection is interrupted.
   // Default implementation does nothing.
   virtual void OnDisconnected();
+
+  // If no app handler has been added with the exact app_id of an incoming
+  // event, all handlers will be asked (in arbitrary order) whether they can
+  // handle the app_id, and the first to return true will receive the event.
+  virtual bool CanHandle(const std::string& app_id);
 };
 
 }  // namespace gcm