[Chromoting] Send DesktopFrame::capturer_id() to client through WebrtcVideoStream

remoting/proto/video_stats.proto

 // Copyright 2016 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.
 
 // Internal message types that should not be seen outside the protocol
 // directory.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package remoting;
 
+// Next Id: 13
 message FrameStatsMessage {
   // Frame ID.
   optional uint32 frame_id = 1;
 
   // Frame size.
   optional int32 frame_size = 2;
 
   // Id of the last event that was injected before this frame was captured.
   optional int64 latest_event_timestamp = 3;
 
@@ skipping 13 matching lines @@
   optional int32 encode_time_ms = 8;
 
   // Time for which the frame is blocked until it's sent to the client.
   optional int32 send_pending_time_ms = 9;
 
   // Current network RTT estimate.
   optional int32 rtt_estimate_ms = 10;
 
   // Current bandwidth estimate in kb/s.
   optional int32 bandwidth_estimate_kbps = 11;
+
+  // The capturer Id to indicate the implementation of ScreenCapturer that
+  // generates this frame.
+  optional uint32 capturer_id = 12;
 }