Implement elementsFromPoint()

[classilla]

https://hg.mozilla.org/releases/mozilla-aurora/rev/530c3a79401a
from https://bugzilla.mozilla.org/show_bug.cgi?id=1164427

https://developer.mozilla.org/en-US/docs/Web/API/DocumentOrShadowRoot/elementsFromPoint

It looks like some sites either depend on it or implement a sluggish polyfill, so this would probably help in a few places, and should apply without pretty much any changes.

[classilla]

This fixes the scrolling menus on Twitch.tv,

[classilla]

Doesn't make much difference to the scrolling issue with Facebook, but works well otherwise, and passes all tests.