title: Google Chromecast subtitle not working
body: <p>I have this code </p>
  
  <pre><code>var englishSubtitle = new chrome.cast.media.Track(2,chrome.cast.media.TrackType.TEXT);
  englishSubtitle.trackContentId = 'english.vtt';
  englishSubtitle.trackContentType = 'text/vtt';
  englishSubtitle.subtype = chrome.cast.media.TextTrackType.CAPTIONS;
  englishSubtitle.name = 'English';
  englishSubtitle.language = 'en-US';
  englishSubtitle.customData = null;
  var tracks = englishSubtitle;
  
  var mediaInfo = new chrome.cast.media.MediaInfo(app.streamState_.manifest);
  mediaInfo.contentType = app.streamState_.type;
  mediaInfo.metadata = new chrome.cast.media.GenericMediaMetadata();
  mediaInfo.customData = null;
  mediaInfo.streamType = chrome.cast.media.StreamType.BUFFERED;
  mediaInfo.textTrackStyle = new chrome.cast.media.TextTrackStyle();
  mediaInfo.tracks = tracks; 
  mediaInfo.metadata.metadataType = chrome.cast.media.MetadataType.GENERIC;
  
  var activeTrackIds = [2];   
  
  var request = new chrome.cast.media.LoadRequest(mediaInfo);
  request.autoplay = true;
  request.currentTime = 0;
  request.activeTrackIds = activeTrackIds;
  session.loadMedia(request,onMediaDiscovered.bind(   this, 'loadedMedia'),  onMediaError);
  </code></pre>
  
  <p>I want to show subtitle on chromecast. When I want to set activeTracks on the request, I receive an error 
      Object {code: "session_error", description: "INVALID_PARAMS", details: Object}
  The subtitle it doesn't show and the video doesn't play it at all, because of that error.
  Am I doing something wrong?</p>