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

components/gcm_driver/gcm_client.h

Index: components/gcm_driver/gcm_client.h
diff --git a/components/gcm_driver/gcm_client.h b/components/gcm_driver/gcm_client.h
index 95a56d9d0e0f667fc24228fcfaaf535447794f88..59b51ff68cf586b6c624f76b6dccc0e0e4e573fc 100644
--- a/components/gcm_driver/gcm_client.h
+++ b/components/gcm_driver/gcm_client.h
@@ -16,6 +16,7 @@
 #include "components/gcm_driver/common/gcm_messages.h"
 #include "components/gcm_driver/crypto/gcm_encryption_provider.h"
 #include "components/gcm_driver/gcm_activity.h"
+#include "components/gcm_driver/gcm_message_status.h"
 #include "components/gcm_driver/registration_info.h"
 
 template <class T> class scoped_refptr;
@@ -177,8 +178,13 @@ class GCMClient {
     // Called when a message has been received.
     // |app_id|: application ID.
     // |message|: message received.
-    virtual void OnMessageReceived(const std::string& app_id,
-                                   const IncomingMessage& message) = 0;
+    // |optional_optional_receipt_callback|: Optional callback to invoke if
+    // desired when processing the message is complete if the receiver wants to
+    // return a delivery receipt. The callback must be invoked on the IO thread.
+    virtual void OnMessageReceived(
+        const std::string& app_id,
+        const IncomingMessage& message,
+        const MessageReceiptCallback& optional_receipt_callback) = 0;
 
     // Called when some messages have been deleted from the server.
     // |app_id|: application ID.