Implement video frame acknowledgements in the chromoting protocol.

remoting/proto/video.proto

 // Copyright (c) 2012 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.
 
 // Protocol for video messages.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
@@ skipping 48 matching lines @@
   // starting to capture this video frame.
   optional int64 latest_event_timestamp = 9;
 
   repeated Rect desktop_shape_rects = 10;
 
   // True when |desktop_shape_rects| should be used.
   optional bool use_desktop_shape = 11;
 
   // Optional frame timestamp. Used in tests to estimate frame latency.
   optional int64 timestamp = 12;
+
+  // Frame identifier used to match VideoFrame and VideoAck.
+  optional int32 frame_id = 13;
 }
+
+// VideoAck acknowledges that the frame in the VideoPacket with the same
+// frame_id has been rendered. VideoAck messages must be sent only for frames
+// that have frame_id field set. They must be sent the same order in which
+// the corresponding VideoPackets were received.
+message VideoAck {
+  // Frame ID.
+  optional int32 frame_id = 1;
+}