setPresentationStartTime(presentationStartTime) {
    goog.asserts.assert(presentationStartTime >= 0,
        'presentationStartTime must be >= 0');
    this.presentationStartTime_ = presentationStartTime;
  }
Sets the presentation's start time.

@param {number} presentationStartTime The wall-clock time, in seconds,
  when the presentation started or will start. Only required for live.
@export