startListeningToSeeks_() {
    goog.asserts.assert(
        this.video_.readyState > 0,
        'The media element should be ready before we listen for seeking.');

    // Now that any startup seeking is complete, we can trust the video element
    // for currentTime.
    this.started_ = true;

    this.eventManager_.listen(this.video_, 'seeking', () => this.onSeek_());
    this.onStarted_(this.video_.currentTime);
  }
Add the listener for seek-events. This will call the externally-provided
|onSeek| callback whenever the media element seeks.

@private