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.
From spec https://drafts.csswg.org/css-grid/#grid-items:
"Each in-flow child of a grid container becomes a grid item, and each
child text sequence is wrapped in an anonymous block container grid
item."
Fixes the problem that text sequences inside grid containers are
ignored and not displayed.