title: canvas going black when using video widevine drm
body: <p>My propose is draw video frame to canvas and draw thing else on canvas. I have test it success in video link without Widevine.</p>
  <pre><code>var canvas = document.getElementById(&quot;canvasDraw&quot;);
  canvas.style.width = 1280;
  canvas.style.height = 720;
  
  var context = canvas.getContext('2d');
  
  var video = document.getElementById('video');
  
  function draw() {
    context.drawImage(video, 0, 0, 1280, 720);
    setTimeout(draw, 1000 / 30);
  }
  video.onplay = function () {
    setTimeout(draw, 1000 / 30);
  }
  </code></pre>
  <p>But when I using widevine drm the frame came out is black but the audio is still playing.</p>
  <p>Can someone help me with this ?</p>
  <p>This is the widevine link I get from shaka-player demo</p>
  <pre><code>https://storage.googleapis.com/shaka-demo-assets/sintel-widevine/dash.mpd
  </code></pre>