MediaQueryList will now remember if a state change occurred when
evaluating its match state. This memory can then be used by the document
later on when it's updating all queries, to ensure that we don't forget
to fire at least one change event.
This also required plumbing the system visibility state to initial
about:blank documents, since otherwise they would be stuck in "hidden"
state indefinitely and never evaluate their media queries.
Spec defines `[LegacyNullToEmptyString]` on `value` argument of
`setProperty` but since `internal_set` calls `setProperty` directly
instead of using IDL generated binding, we need to make sure that null
is treated as empty string.
Fixes items grid loading on https://d.rsms.me/stuff/
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.
