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..facc706ed9a09eb592d4b6cac160b6d0d1181ee4 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();
 
-  webm_muxer_.reset(
-      new media::WebmMuxer(use_vp9_ ? media::kCodecVP9 : media::kCodecVP8,
-                           base::Bind(&MediaRecorderHandler::WriteData,
-                                      weak_factory_.GetWeakPtr())));
-
   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 - http://crbug.com/528519 */,
+      base::Bind(&MediaRecorderHandler::WriteData,
+                 weak_factory_.GetWeakPtr())));
+
   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