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.
Properties like min-width, max-width, etc, should be ignored while we're
trying to determine the intrinsic size of a flex container.
This fixes an infinite recursion when using an intrinsic size keyword as
the max-width of a flex column container.
Note that this behavior is marked as AD-HOC in code comments because
specs don't tell us how to achieve intrinsic sizing.
We can now load product pages on the Twinings site, such as
https://twinings.co.uk/products/earl-grey-100-tea-bags :^)