static getInitDataClearKey_(element, keyIds) {
    if (keyIds.size == 0) {
      return null;
    }

    const systemId = new Uint8Array([
      0x10, 0x77, 0xef, 0xec, 0xc0, 0xb2, 0x4d, 0x02,
      0xac, 0xe3, 0x3c, 0x1e, 0x52, 0xe2, 0xfb, 0x4b,
    ]);
    const data = new Uint8Array([]);
    const psshVersion = 1;
    const pssh =
         shaka.util.Pssh.createPssh(data, systemId, keyIds, psshVersion);

    return [
      {
        initData: pssh,
        initDataType: 'cenc',
        keyId: element.keyId,
      },
    ];
  }
Creates ClearKey initData from Default_KID value retrieved from previously
parsed ContentProtection tag.
@param {shaka.dash.ContentProtection.Element} element
@param {!Set<string>} keyIds
@return {?Array<shaka.extern.InitDataOverride>}
@private