async _takeScreenshot(context) {
    const [metrics, result] = await Promise.all([
      context.driver.defaultSession.sendCommand('Page.getLayoutMetrics'),
      context.driver.defaultSession.sendCommand('Page.captureScreenshot', {
        format: FULL_PAGE_SCREENSHOT_FORMAT,
        quality: FULL_PAGE_SCREENSHOT_QUALITY,
      }),
    ]);
    const data = `data:image/${FULL_PAGE_SCREENSHOT_FORMAT};base64,` + result.data;

    return {
      data,
      width: metrics.cssVisualViewport.clientWidth,
      height: metrics.cssVisualViewport.clientHeight,
    };
  }
@param {LH.Gatherer.Context} context
@return {Promise<LH.Result.FullPageScreenshot['screenshot']>}