Allow the IntersectionObserver constructor to take a Document as the 'root' argument, causing intersections to be calculated against the scrolling viewport of the document.
This is primarily targeted towards observers running in an iframe; prior to this feature, there was no way to measure intersection with the scrolling viewport of the iframe's document.
Docs: https://w3c.github.io/IntersectionObserver/#calculate-intersection-rect-algo
Explainers: https://github.com/w3c/IntersectionObserver/issues/372