This is more consistent with what the Rust bits of the style system do, and
removes a pointer from ComputedStyle which is always nice.
This also aligns the Rust bits with the C++ bits re. not treating xul pseudos as
anonymous boxes. See the comment in nsTreeStyleCache.cpp regarding those.
Can't wait for XUL trees to die.
Depends on D19001
Differential Revision: https://phabricator.services.mozilla.com/D19002
--HG--
extra : moz-landing-system : lando