Before this change, we were doing it after every layout, which meant
that already-propagated overflow could be propagated again, which led to
incorrect scrolling behavior.
This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
When embedding an SVG in an img element, if the external SVG's root
element has both width and height attributes, but no viewBox attribute,
we now create a fallback viewBox with "0 0 width height".
This appears to match the behavior of other browsers. Inspired by
discussion on Mozilla's bug tracker:
https://bugzilla.mozilla.org/show_bug.cgi?id=614649
