Provide a mechanism for the GCM driver to send message receipts to GCM.

components/gcm_driver/gcm_driver_desktop.h

Index: components/gcm_driver/gcm_driver_desktop.h
diff --git a/components/gcm_driver/gcm_driver_desktop.h b/components/gcm_driver/gcm_driver_desktop.h
index cdd0ac79a685c8064adf84eb63b25a56367ee674..631c9bd8e122c727dcc32f576c9d8019c410b273 100644
--- a/components/gcm_driver/gcm_driver_desktop.h
+++ b/components/gcm_driver/gcm_driver_desktop.h
@@ -157,7 +157,8 @@ class GCMDriverDesktop : public GCMDriver,
 
   // Callbacks posted from IO thread to UI thread.
   void MessageReceived(const std::string& app_id,
-                       const IncomingMessage& message);
+                       const IncomingMessage& message,
+                       const MessageReceiptCallback& callback);
   void MessagesDeleted(const std::string& app_id);
   void MessageSendError(const std::string& app_id,
                         const GCMClient::SendErrorDetails& send_error_details);
@@ -183,6 +184,9 @@ class GCMDriverDesktop : public GCMDriver,
                            const std::string& scope,
                            GCMClient::Result result);
 
+  void DoSendMessageReceipt(const MessageReceiptCallback& callback,
+                            GCMMessageStatus status) override;
+
   std::unique_ptr<GCMChannelStatusSyncer> gcm_channel_status_syncer_;
 
   // Flag to indicate whether the user is signed in to a GAIA account.