_onStylesheetAdded(event) {
    if (!this._session) throw new Error('Session not initialized');
    const styleSheetId = event.header.styleSheetId;
    const sheetPromise = this._session.sendCommand('CSS.getStyleSheetText', {styleSheetId})
      .then(content => ({
        header: event.header,
        content: content.text,
      }))
      .catch(err => {
        log.warn(
          'Stylesheets',
          `Error fetching content of stylesheet with URL "${event.header.sourceURL}"`
        );
        Sentry.captureException(err, {
          tags: {
            gatherer: 'Stylesheets',
          },
          extra: {
            url: event.header.sourceURL,
          },
          level: 'error',
        });
        return null;
      });
    this._sheetPromises.set(styleSheetId, sheetPromise);
  }
@param {LH.Crdp.CSS.StyleSheetAddedEvent} event