_loadInLegacyViewerVersion(reportJson) {
    const warnMsg = `Version mismatch between viewer and JSON. Opening compatible viewer...`;
    logger.log(warnMsg, false);

    // Place report in IDB, then navigate current tab to the legacy viewer
    const viewerPath = new URL('../viewer2x/', location.href);
    idbKeyval.set('2xreport', reportJson).then(_ => {
      window.location.href = viewerPath.href;
    });
  }
Stores v2.x report in IDB, then navigates to legacy viewer in current tab.
@param {LH.Result} reportJson
@private