static installFullPageScreenshot(el, screenshot) {
    el.style.setProperty('--element-screenshot-url', `url('${screenshot.data}')`);
  }
Called by report renderer. Defines a css variable used by any element screenshots
in the provided report element.
Allows for multiple Lighthouse reports to be rendered on the page, each with their
own full page screenshot.
@param {HTMLElement} el
@param {LH.Result.FullPageScreenshot['screenshot']} screenshot