Implemented Multiple video track recoding.

content/renderer/media/media_recorder_handler.cc

Index: content/renderer/media/media_recorder_handler.cc
diff --git a/content/renderer/media/media_recorder_handler.cc b/content/renderer/media/media_recorder_handler.cc
index 5934ef9fd11ee121a05e63fa12dcb506736b2ce8..b9e853fe87122c49534f002291f3b3c797670427 100644
--- a/content/renderer/media/media_recorder_handler.cc
+++ b/content/renderer/media/media_recorder_handler.cc
@@ -75,22 +75,21 @@ bool MediaRecorderHandler::start(int timeslice) {
     LOG(WARNING) << "Recording no video tracks is not implemented";
     return false;
   }
-  // TODO(mcasas): The muxer API supports only one video track. Extend it to
-  // several video tracks, see http://crbug.com/528523.
-  LOG_IF(WARNING, video_tracks.size() > 1u) << "Recording multiple video"
-      << " tracks is not implemented.  Only recording first video track.";
-  const blink::WebMediaStreamTrack& video_track = video_tracks[0];
-  if (video_track.isNull())
-    return false;
-
-  const VideoTrackRecorder::OnEncodedVideoCB on_encoded_video_cb =
-      base::Bind(&media::WebmMuxer::OnEncodedVideo,
-                 base::Unretained(webm_muxer_.get()));
 
-  video_recorders_.push_back(new VideoTrackRecorder(video_track,
-                                                    on_encoded_video_cb));
+  for (const blink::WebMediaStreamTrack& video_track : video_tracks) {
+    if (video_track.isNull()) {
+      LOG(WARNING) << "|video_track| is null";
+      continue;
+    }
+    const VideoTrackRecorder::OnEncodedVideoCB on_encoded_video_cb =
+        base::Bind(&media::WebmMuxer::OnEncodedVideo,
+                   base::Unretained(webm_muxer_.get()),
+                   webm_muxer_->GetNextTrackIndex());
+    video_recorders_.push_back(
+        new VideoTrackRecorder(video_track, on_encoded_video_cb));
+    recording_ = true;
+  }
 
-  recording_ = true;
   return true;
 }