function screenshot() {
  return this.requestHandler.create('/session/:sessionId/screenshot');
}
Take a screenshot of the current viewport. To get the screenshot of the whole page
use the action command `saveScreenshot`

@return {String} screenshot   The screenshot as a base64 encoded PNG.

@see  https://w3c.github.io/webdriver/webdriver-spec.html#take-screenshot
@type protocol