boxes unless we're determining the style context parent for something that is
itself a css anon box (and is not a non-element frame). Fixes bug 323656
(which is where the patch is), bug 85872, bug 280610. As far as I can tell,
also fixes bug 317876, bug 372376, bug 374297. r+sr=dbaron
git-svn-id: svn://10.0.0.236/trunk@224561 18797224-902f-48f8-a5cc-f745e15eee43