_renderFlowResult(json, rootEl, saveGistCallback) {
    // TODO: Add save HTML functionality with ReportGenerator loaded async.
    renderFlowReport(json, rootEl, {
      saveAsGist: saveGistCallback,
    });
    // Install as global for easier debugging.
    window.__LIGHTHOUSE_FLOW_JSON__ = json;
    // eslint-disable-next-line no-console
    console.log('window.__LIGHTHOUSE_FLOW_JSON__', json);
  }
@param {LH.FlowResult} json
@param {HTMLElement} rootEl
@param {(json: LH.Result|LH.FlowResult) => void} [saveGistCallback]