getMetadataTracks_() {
    goog.asserts.assert(this.video_.textTracks,
        'TextTracks should be valid.');
    return Array.from(this.video_.textTracks)
        .filter((t) => t.kind == 'metadata');
  }
Get the TextTracks with the 'metadata' kind.

@return {!Array<TextTrack>}
@private