Bug 1525955 - Include anon boxes in CSSPseudoElementType, to remove ComputedStyle::mPseudoTag. r=heycam

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