onStarted(startTime) {
    if (this.video_.seeking && !this.seekingEventReceived_) {
      this.seekingEventReceived_ = true;
      this.startTime_ = startTime;
    }
    if (this.gapJumpTimer_) {
      this.gapJumpTimer_.tickEvery(this.config_.gapJumpTimerTime);
    }
    this.onPollGapJump_();
  }
Called when playback has started and the video element is
listening for seeks.

@param {number} startTime