async getArtifact(passContext) {
    const {finalDisplayedUrl} = passContext.baseArtifacts.URL;
    const robotsUrl = new URL('/robots.txt', finalDisplayedUrl).href;
    return passContext.driver.fetcher.fetchResource(robotsUrl)
      .catch(err => ({status: null, content: null, errorMessage: err.message}));
  }
@param {LH.Gatherer.Context} passContext
@return {Promise<LH.Artifacts['RobotsTxt']>}