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

remoting/proto/video_stats.proto

Index: remoting/proto/video_stats.proto
diff --git a/remoting/proto/video_stats.proto b/remoting/proto/video_stats.proto
index 198edcab5bfc75bdbff0fc0913e76fa17fcf887b..6bc4bb639d727032a971144aa9e7e70a10cb7aca 100644
--- a/remoting/proto/video_stats.proto
+++ b/remoting/proto/video_stats.proto
@@ -11,6 +11,7 @@ option optimize_for = LITE_RUNTIME;
 
 package remoting;
 
+// Next Id: 13
 message FrameStatsMessage {
   // Frame ID.
   optional uint32 frame_id = 1;
@@ -44,5 +45,9 @@ message FrameStatsMessage {
 
   // 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;
 }