ver has more than 2 concurrent
   * in-flight network requests. Interactive 'candidate' is defined as end of
   * the last long task before this quiet window, or as |searchBegin| if there
   * is no long task before that window. We define Interactive as max of
   * Interactive 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 Pai