My previous patch somehow causes WR to crash and fall back to d2d on Windows,
see bug 1671591.
While I figure that out let's fix TB by restoring the previous behavior
for XUL.
Rename the Root frame type because it's super-confusing that IsRootFrame
is not equivalent to !GetParent().
Differential Revision: https://phabricator.services.mozilla.com/D94105