static calculateResponseTime(record) {
    // Lightrider does not have timings for sendEnd, but we do have this timing which should be
    // close to the response time.
    if (global.isLightrider && record.lrStatistics) return record.lrStatistics.requestMs;

    if (!record.timing) return null;
    return record.timing.receiveHeadersStart - record.timing.sendEnd;
  }
@param {LH.Artifacts.NetworkRequest} record
@return {number|null}