turns {number|undefined}
   */
  function findFirstCpuIdleTime(searchBegin, searchEnd,
      domContentLoadedEnd, longTasksInWindow) {
    const sortedLongTasks = longTasksInWindow.sort((a, b) => a.start - b.start);
    const taskClusters = findFCITaskClusters(sortedLongTasks);
    const heavyTaskClusters = taskClusters.filter(cluster => cluster.isHeavy());

    let quietWindowBegin = searchBegin;
    let fiCandidate = undefined;
    for (const cluster of heavyTaskClusters) {
      if (reachedFCIQuiescence_