pssh_(streamInfo) {
    const initDatas = [];
    if (!streamInfo.encrypted) {
      return new Uint8Array([]);
    }

    for (const drmInfo of streamInfo.stream.drmInfos) {
      if (!drmInfo.initData) {
        continue;
      }
      for (const initData of drmInfo.initData) {
        initDatas.push(initData.initData);
      }
    }
    const boxes = shaka.util.Uint8ArrayUtils.concat(...initDatas);
    return boxes;
  }
Generate a PSSH box

@param {shaka.util.Mp4Generator.StreamInfo} streamInfo
@return {!Uint8Array}
@private