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

components/gcm_driver/gcm_client_impl.h

Index: components/gcm_driver/gcm_client_impl.h
diff --git a/components/gcm_driver/gcm_client_impl.h b/components/gcm_driver/gcm_client_impl.h
index 9e102154b86b20e1cbe8c7ac5d81432a36087b1f..8c3ea6737bc36661b84603ef3f50ca6be585f2be 100644
--- a/components/gcm_driver/gcm_client_impl.h
+++ b/components/gcm_driver/gcm_client_impl.h
@@ -51,6 +51,7 @@ namespace gcm {
 class CheckinRequest;
 class ConnectionFactory;
 class GCMClientImplTest;
+enum class GCMMessageStatus;
 
 // Helper class for building GCM internals. Allows tests to inject fake versions
 // as necessary.
@@ -323,6 +324,13 @@ class GCMClientImpl
   // Reset all cahced values.
   void ResetCache();
 
+  // Send a message to GCM with information about the final status of a
+  // previously received message. This can be run as a result of a callback
+  // being executed from an AppHandler.
+  void SendMessageReceipt(const std::string& message_id,
+                          const std::string& app_id,
+                          GCMMessageStatus status);
+
   // Builder for the GCM internals (mcs client, etc.).
   std::unique_ptr<GCMInternalsBuilder> internals_builder_;