static convertPeriod_(old, periodDuration) {
    const V1StorageCell = shaka.offline.indexeddb.V1StorageCell;

    // In the case that this is really old (like really old, like dinosaurs
    // roaming the Earth old) there may be no variants, so we need to add those.
    V1StorageCell.fillMissingVariants_(old);

    for (const stream of old.streams) {
      const message = 'After filling in missing variants, ' +
                      'each stream should have variant ids';
      goog.asserts.assert(stream.variantIds, message);
    }

    return old.streams.map((stream) => V1StorageCell.convertStream_(
        stream, old.startTime, periodDuration));
  }
@param {shaka.extern.PeriodDBV1} old
@param {number} periodDuration
@return {!Array<shaka.extern.StreamDB>}
@private