function getChromeVersionString() {
    const userAgent = lhr.environment.hostUserAgent;
    const userAgentMatch = /Chrome\/([\d.]+)/.exec(userAgent); // Chrome/85.0.4174.0
    if (!userAgentMatch) throw new Error('Could not get chrome version.');
    const versionString = userAgentMatch[1];
    if (versionString.split('.').length !== 4) throw new Error(`unexpected ua: ${userAgent}`);
    return versionString;
  }
Lazily compute the Chrome version because some reports are explicitly asserting error conditions.
@returns {string}