createPlayhead(startTime) {
    goog.asserts.assert(this.manifest_, 'Must have manifest');
    goog.asserts.assert(this.video_, 'Must have video');
    return new shaka.media.MediaSourcePlayhead(
        this.video_,
        this.manifest_,
        this.config_.streaming,
        startTime,
        () => this.onSeek_(),
        (event) => this.dispatchEvent(event));
  }
Creates a new instance of Playhead.  This can be replaced by tests to
create fake instances instead.

@param {?number|Date} startTime
@return {!shaka.media.Playhead}