the time
   * interval from the beginning of first task to the end of last task) is
   * longer than 250ms. We call the beginning of the quiet window FCI candidate,
   * and define First Cpu Idle as max of FCI candidate and
   * domContentLoadedEnd.
   *
   * Returns undefined if no suitable quiet window is found.
   *
   * @param {number} searchBegin - The left boundary for our search for quiet
   *     window. Ideally, this is First Meaningful Paint, but if FMP is not
   *     available, First Contentful 