[GCM] Adding the OnSendAcknowledgement event

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 6fce669f450749fc7dd63b55f39b5b752fe8b6ff..e5082c59080d08159864af512aac57b67a725387 100644
--- a/components/gcm_driver/gcm_client.h
+++ b/components/gcm_driver/gcm_client.h
@@ -184,6 +184,12 @@ class GCMClient {
         const std::string& app_id,
         const SendErrorDetails& send_error_details) = 0;
 
+    // Called when a message was acknowledged by the GCM server.
+    // |app_id|: application ID.
+    // |message_id|: ID of the acknowledged message.
+    virtual void OnSendAcknowledged(const std::string& app_id,
+                                    const std::string& message_id) = 0;
+
     // Called when the GCM becomes ready. To get to this state, GCMClient
     // finished loading from the GCM store and retrieved the device check-in
     // from the server if it hadn't yet.