getLiveLatency() {
    if (!this.video_ || !this.video_.currentTime) {
      return null;
    }
    const now = this.getPresentationStartTimeAsDate().getTime() +
      this.video_.currentTime * 1000;

    return Math.floor(Date.now() - now);
  }
Get latency in milliseconds between the live edge and what's currently
playing.

@return {?number} The latency in milliseconds, or null if nothing
is playing.