async _onSaveJson(reportJson) {
    if (window.gtag) {
      window.gtag('event', 'report', {type: 'share'});
    }

    // TODO: find and reuse existing json gist if one exists.
    try {
      const id = await this._github.createGist(reportJson);
      if (window.gtag) {
        window.gtag('event', 'report', {type: 'created'});
      }
      history.pushState({}, '', `${LighthouseReportViewer.APP_URL}?gist=${id}`);
      return id;
    } catch (err) {
      logger.log(err.message);
    }
  }
Saves the current report by creating a gist on GitHub.
@param {LH.Result|LH.FlowResult} reportJson
@return {Promise<string|void>} id of the created gist.
@private