fromInitSegmentDB_(key) {
    /** @type {!shaka.offline.OfflineUri} */
    const uri = shaka.offline.OfflineUri.segment(
        this.mechanism_, this.cell_, key);

    return new shaka.media.InitSegmentReference(
        () => [uri.toString()],
        /* startBytes= */ 0,
        /* endBytes= */ null );
  }
@param {number} key
@return {!shaka.media.InitSegmentReference}
@private