   not in the middle of a heavy task cluster.
   * @param {number} mainThreadQuietWindowStart - Most recent timestamp when we
   *     considered the main thread to be quiet. Usually end of most recent
   *     heavy task cluster or |searchBegin| where there are no heavy task
   *     cluster.
   * @param {number} searchBegin - Left boundary for quiet window search.
   * @returns {boolean}
   */
  function reachedFCIQuiescence_(timestamp, mainThreadQuietWindowStart,
      searchBegin) {
    const quietWindo