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

components/gcm_driver/common/gcm_messages.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_GCM_DRIVER_COMMON_GCM_MESSAGES_H_
 #define COMPONENTS_GCM_DRIVER_COMMON_GCM_MESSAGES_H_
 
 #include <map>
 #include <string>
 
@@ skipping 18 matching lines @@
 
   static const int kMaximumTTL;
 };
 
 // Message being received from the other party.
 struct GCM_DRIVER_EXPORT IncomingMessage {
   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;
   std::string raw_data;
 
   // Whether the contents of the message have been decrypted, and are
   // available in |raw_data|.
   bool decrypted;
 };
 
 }  // namespace gcm
 
 #endif  // COMPONENTS_GCM_DRIVER_COMMON_GCM_MESSAGES_H_