function buildInitData(base64Psshs, keyIds = []) {
    return base64Psshs.map((base64, index) => {
      /** @type {shaka.extern.InitDataOverride} */
      const initData = {
        initDataType: 'cenc',
        initData: shaka.util.Uint8ArrayUtils.fromBase64(base64),
        keyId: keyIds[index] || null,
      };
      return initData;
    });
  }
Build an expected InitDataOverride based on base-64-encoded PSSHs and
optional key IDs.

@param {!Array<string>} base64Psshs
@param {!Array<string>=} keyIds
@return {!Array<shaka.extern.InitDataOverride>}