updateEvery(interval, updateCallback) {
    goog.asserts.assert(!this.timer_, 'SegmentIndex timer already started!');
    if (this.immutable_) {
      return;
    }
    if (this.timer_) {
      this.timer_.stop();
    }

    this.timer_ = new shaka.util.Timer(() => {
      const references = updateCallback();
      if (references) {
        this.references.push(...references);
      } else {
        this.timer_.stop();
        this.timer_ = null;
      }
    });
    this.timer_.tickEvery(interval);
  }
Updates the references every so often.  Stops when the references list
returned by the callback is null.

@param {number} interval The interval in seconds.
@param {function(): Array<shaka.media.SegmentReference>} updateCallback
@export