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..f6139e0724dd1979490c564b4b87ee1281b61034 100644
--- a/components/gcm_driver/gcm_driver_desktop.h
+++ b/components/gcm_driver/gcm_driver_desktop.h
@@ -155,9 +155,14 @@ class GCMDriverDesktop : public GCMDriver,
                      const std::string& authorized_entity,
                      const std::string& scope);
 
+  void DoSendMessageReceipt(
+      const MessageReceiptCallback& optional_receipt_callback,
+      GCMMessageStatus status);
+
   // Callbacks posted from IO thread to UI thread.
   void MessageReceived(const std::string& app_id,
-                       const IncomingMessage& message);
+                       const IncomingMessage& message,
+                       const MessageReceiptCallback& optional_receipt_callback);
   void MessagesDeleted(const std::string& app_id);
   void MessageSendError(const std::string& app_id,
                         const GCMClient::SendErrorDetails& send_error_details);