title: Media Recorder API generated webm badly encoded?
body: <p>when I record a video with the Media Recorder API and download &amp; open it with VLC, length of the video is unknow.</p>
  
  <p>Furthermore, on Chrome mobile (52.0.2743.98) if I read it in a video tag, length isn't show, currentTime will always be 0 and the ontimeupdate will only launch when the video end.</p>
  
  <p>On Chrome desktop (52.0.2743.116), length isn't show but ontimeupdate and currentTime seem to work fine.</p>
  
  <p>I've recorded and downloaded the video from the <a href="https://webrtc.github.io/samples/src/content/getusermedia/record/" rel="noreferrer">developers google demo</a>.
  The video is a webm as It seem to be the only type <a href="https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/fast/mediarecorder/MediaRecorder-isTypeSupported.html" rel="noreferrer">supported by chrome</a></p>
  
  <p>Any idea ?</p>
  
  <p>Metadata from ffpmeg :</p>
  
  <p><code>
    libavutil      54. 31.100 / 54. 31.100
    libavcodec     56. 60.100 / 56. 60.100
    libavformat    56. 40.101 / 56. 40.101
    libavdevice    56.  4.100 / 56.  4.100
    libavfilter     5. 40.101 /  5. 40.101
    libswscale      3.  1.101 /  3.  1.101
    libswresample   1.  2.101 /  1.  2.101
    libpostproc    53.  3.100 / 53.  3.100
  Input #0, matroska,webm, from 'test.webm':
    Metadata:
      encoder         : Chrome
    Duration: N/A, start: 0.000000, bitrate: N/A
      Stream #0:0(eng): Audio: opus, 48000 Hz, mono, fltp (default)
      Stream #0:1(eng): Video: vp9 (Profile 0), yuv420p(tv), 480x640, SAR 1:1 DAR
  3:4, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
  File 'metadata.txt' already exists. Overwrite ? [y/N] y
  Output #0, ffmetadata, to 'metadata.txt':
    Metadata:
      encoder         : Lavf56.40.101
  Stream mapping:
  size=       0kB time=--577014:-32:-22.-77 bitrate=N/A
  video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
  </code></p>