sendInProgressEvents_(currentTime, ad) {
    const MediaTailorAdManager = shaka.ads.MediaTailorAdManager;
    const firstQuartileTime = ad.startTimeInSeconds +
        0.25 * ad.durationInSeconds;
    const midpointTime = ad.startTimeInSeconds +
        0.5 * ad.durationInSeconds;
    const thirdQuartileTime = ad.startTimeInSeconds +
        0.75 * ad.durationInSeconds;
    if (currentTime >= firstQuartileTime &&
        !this.eventsSent.includes(MediaTailorAdManager.FIRSTQUARTILE_)) {
      this.eventsSent.push(MediaTailorAdManager.FIRSTQUARTILE_);
      this.sendTrackingEvent_(MediaTailorAdManager.FIRSTQUARTILE_);
    } else if (currentTime >= midpointTime &&
        !this.eventsSent.includes(MediaTailorAdManager.MIDPOINT_)) {
      this.eventsSent.push(MediaTailorAdManager.MIDPOINT_);
      this.sendTrackingEvent_(MediaTailorAdManager.MIDPOINT_);
    } else if (currentTime >= thirdQuartileTime &&
        !this.eventsSent.includes(MediaTailorAdManager.THIRDQUARTILE_)) {
      this.eventsSent.push(MediaTailorAdManager.THIRDQUARTILE_);
      this.sendTrackingEvent_(MediaTailorAdManager.THIRDQUARTILE_);
    }
  }
@param {number} currentTime
@param {mediaTailor.Ad} ad
@private