getSegmentAvailabilityStart() {
    goog.asserts.assert(this.segmentAvailabilityDuration_ >= 0,
        'The availability duration should be positive');

    const end = this.getSegmentAvailabilityEnd();
    const start = end - this.segmentAvailabilityDuration_;
    return Math.max(this.userSeekStart_, start);
  }
Gets the presentation's current segment availability start time.  Segments
ending at or before this time should be assumed to be unavailable.

@return {number} The current segment availability start time, in seconds,
  relative to the start of the presentation.
@export