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