Update WebmMuxer for audio component of MediaStream recording.

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 e5c8ccfb6d530019dbab4d8afd61803d14e0b9ee..a8d853b89eedda3f97dd556e14074cd8e48787ca 100644
--- a/content/renderer/media/media_recorder_handler.cc
+++ b/content/renderer/media/media_recorder_handler.cc
@@ -80,14 +80,15 @@ bool MediaRecorderHandler::start(int timeslice) {
   timeslice_ = TimeDelta::FromMilliseconds(timeslice);
   slice_origin_timestamp_ = TimeTicks::Now();
 
+  blink::WebVector<blink::WebMediaStreamTrack> video_tracks;
+  media_stream_.videoTracks(video_tracks);
+
   webm_muxer_.reset(
       new media::WebmMuxer(use_vp9_ ? media::kCodecVP9 : media::kCodecVP8,
+                           video_tracks.size(), 0 /* no audio for now */,
                            base::Bind(&MediaRecorderHandler::WriteData,
                                       weak_factory_.GetWeakPtr())));
 
-  blink::WebVector<blink::WebMediaStreamTrack> video_tracks;
-  media_stream_.videoTracks(video_tracks);
-
   if (video_tracks.isEmpty()) {
     // TODO(mcasas): Add audio_tracks and update the code in this function
     // correspondingly, see http://crbug.com/528519. As of now, only video