updateInitSegmentReference(initSegmentReference) {
    if (this.initSegmentReference_ === initSegmentReference) {
      return;
    }

    this.initSegmentReference_ = initSegmentReference;
    for (const reference of this.references) {
      if (reference) {
        reference.updateInitSegmentReference(initSegmentReference);
      }
    }
  }
Updates the init segment reference and propagates the update to all
references.
@param {shaka.media.InitSegmentReference} initSegmentReference