static getNetworkUserAgent(devtoolsLog) {
    for (const entry of devtoolsLog) {
      if (entry.method !== 'Network.requestWillBeSent') continue;
      const userAgent = entry.params.request.headers['User-Agent'];
      if (userAgent) return userAgent;
    }

    return '';
  }
@param {LH.Artifacts['DevtoolsLog']} devtoolsLog
@return {string}