[GCM] Add basic collapse key support for upstream

google_apis/gcm/engine/mcs_client.h

Index: google_apis/gcm/engine/mcs_client.h
diff --git a/google_apis/gcm/engine/mcs_client.h b/google_apis/gcm/engine/mcs_client.h
index 036acb8b159f2079b2f3818e3cef2296ebae274e..52eaa0cf46d4125734a78cb200bbf1c3d4fb2c38 100644
--- a/google_apis/gcm/engine/mcs_client.h
+++ b/google_apis/gcm/engine/mcs_client.h
@@ -35,6 +35,7 @@ class LoginRequest;
 
 namespace gcm {
 
+class CollapseKey;
 class ConnectionFactory;
 struct ReliablePacketInfo;
 
@@ -190,6 +191,10 @@ class GCM_EXPORT MCSClient {
   void NotifyMessageSendStatus(const google::protobuf::MessageLite& protobuf,
                                MessageSendStatus status);
 
+  // Pops the next message from the front of the send queue (cleaning up
+  // any associated state).
+  MCSPacketInternal PopMessageForSend();
+
   // Clock for enforcing TTL. Passed in for testing.
   base::Clock* const clock_;
 
@@ -220,6 +225,9 @@ class GCM_EXPORT MCSClient {
   std::deque<MCSPacketInternal> to_send_;
   std::deque<MCSPacketInternal> to_resend_;
 
+  // Map of collapse keys to their pending messages.
+  std::map<CollapseKey, ReliablePacketInfo*> collapse_key_map_;
+
   // Last device_to_server stream id acknowledged by the server.
   StreamId last_device_to_server_stream_id_received_;
   // Last server_to_device stream id acknowledged by this device.