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

components/gcm_driver/common/gcm_messages.h

Index: components/gcm_driver/common/gcm_messages.h
diff --git a/components/gcm_driver/common/gcm_messages.h b/components/gcm_driver/common/gcm_messages.h
index 24be478e798f93195fc62a5ac017b9a73ff09501..d4328151875c62776a2fbf93ad7accbcfd4d3cc9 100644
--- a/components/gcm_driver/common/gcm_messages.h
+++ b/components/gcm_driver/common/gcm_messages.h
@@ -36,6 +36,9 @@ struct GCM_DRIVER_EXPORT IncomingMessage {
   IncomingMessage(const IncomingMessage& other);
   ~IncomingMessage();
 
+  // Message ID.
+  std::string id;

[Peter Beverloo, 2016/12/15 19:24:24]

I'd very much prefer not to expose the message ID if we can avoid it - it's an
entirely internal implementation detail. Is there any way we can bind it to the
callback as opposed to stashing it in here?

[harkness, 2016/12/21 17:23:37]

Now that the callback is created in the GCMClient, I could remove this.

Done.

+
   MessageData data;
   std::string collapse_key;
   std::string sender_id;