static _findNetworkQuietPeriods(networkRecords, processedNavigation) {
    const traceEndTsInMs = processedNavigation.timestamps.traceEnd / 1000;
    // Ignore records that failed, never finished, or were POST/PUT/etc.
    const filteredNetworkRecords = networkRecords.filter(record => {
      return record.finished && record.requestMethod === 'GET' && !record.failed &&
          // Consider network records that had 4xx/5xx status code as "failed"
          record.statusCode < 400;
    });
    return NetworkMonitor.findNetworkQuietPeriods(filteredNetworkRecords,
      ALLOWED_CONCURRENT_REQUESTS, traceEndTsInMs);
  }
Finds all time periods where the number of inflight requests is less than or equal to the
number of allowed concurrent requests (2).
@param {Array<LH.Artifacts.NetworkRequest>} networkRecords
@param {{timestamps: {traceEnd: number}}} processedNavigation
@return {Array<TimePeriod>}