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

components/gcm_driver/gcm_driver_desktop.cc

Index: components/gcm_driver/gcm_driver_desktop.cc
diff --git a/components/gcm_driver/gcm_driver_desktop.cc b/components/gcm_driver/gcm_driver_desktop.cc
index 599957d2a5979fced996c2440031f4cee4344b7c..c896d948a3cf44c1abf576ae266dcc5edc5d31cd 100644
--- a/components/gcm_driver/gcm_driver_desktop.cc
+++ b/components/gcm_driver/gcm_driver_desktop.cc
@@ -53,8 +53,10 @@ class GCMDriverDesktop::IOWorker : public GCMClient::Delegate {
   void OnSendFinished(const std::string& app_id,
                       const std::string& message_id,
                       GCMClient::Result result) override;
-  void OnMessageReceived(const std::string& app_id,
-                         const IncomingMessage& message) override;
+  void OnMessageReceived(
+      const std::string& app_id,
+      const IncomingMessage& message,
+      const GCMClient::MessageReceiptCallback& callback) override;
   void OnMessagesDeleted(const std::string& app_id) override;
   void OnMessageSendError(
       const std::string& app_id,
@@ -233,15 +235,13 @@ void GCMDriverDesktop::IOWorker::OnSendFinished(const std::string& app_id,
 
 void GCMDriverDesktop::IOWorker::OnMessageReceived(
     const std::string& app_id,
-    const IncomingMessage& message) {
+    const IncomingMessage& message,
+    const GCMClient::MessageReceiptCallback& callback) {
   DCHECK(io_thread_->RunsTasksOnCurrentThread());
 
-  ui_thread_->PostTask(
-      FROM_HERE,
-      base::Bind(&GCMDriverDesktop::MessageReceived,
-                 service_,
-                 app_id,
-                 message));
+  ui_thread_->PostTask(FROM_HERE,
+                       base::Bind(&GCMDriverDesktop::MessageReceived, service_,
+                                  app_id, message, callback));
 }
 
 void GCMDriverDesktop::IOWorker::OnMessagesDeleted(const std::string& app_id) {
@@ -1215,15 +1215,17 @@ void GCMDriverDesktop::RemoveCachedData() {
   ClearCallbacks();
 }
 
-void GCMDriverDesktop::MessageReceived(const std::string& app_id,
-                                       const IncomingMessage& message) {
+void GCMDriverDesktop::MessageReceived(
+    const std::string& app_id,
+    const IncomingMessage& message,
+    const GCMClient::MessageReceiptCallback& callback) {
   DCHECK(ui_thread_->RunsTasksOnCurrentThread());
 
   // Drop the event if the service has been stopped.
   if (!gcm_started_)
     return;
 
-  DispatchMessage(app_id, message);
+  DispatchMessage(app_id, message, callback);
 }
 
 void GCMDriverDesktop::MessagesDeleted(const std::string& app_id) {