style: Simplify invalid custom property handling.
style: Simplify invalid custom property handling.

It's a bit useless to keep a set of invalid properties if we're going
to use them just to reject lookups into another key. This makes it more
consistent with the cascade / no-references code, and should not change
behavior.

Differential Revision: https://phabricator.services.mozilla.com/D9632

peterjoel/servo,notriddle/servo,paulrouget/servo,pyfisch/servo,pyfisch/servo,DominoTree/servo,peterjoel/servo,jimberlage/servo,DominoTree/servo,szeged/servo,splav/servo,szeged/servo,notriddle/servo,KiChjang/servo,DominoTree/servo,splav/servo,pyfisch/servo,splav/servo,pyfisch/servo,KiChjang/servo,splav/servo,paulrouget/servo,peterjoel/servo,saneyuki/servo,paulrouget/servo,saneyuki/servo,larsbergstrom/servo,paulrouget/servo,peterjoel/servo,saneyuki/servo,paulrouget/servo,notriddle/servo,KiChjang/servo,KiChjang/servo,paulrouget/servo,pyfisch/servo,DominoTree/servo,peterjoel/servo,splav/servo,splav/servo,peterjoel/servo,saneyuki/servo,saneyuki/servo,peterjoel/servo,pyfisch/servo,saneyuki/servo,szeged/servo,nnethercote/servo,KiChjang/servo,nnethercote/servo,peterjoel/servo,KiChjang/servo,nnethercote/servo,paulrouget/servo,splav/servo,jimberlage/servo,emilio/servo,DominoTree/servo,emilio/servo,szeged/servo,emilio/servo,larsbergstrom/servo,larsbergstrom/servo,szeged/servo,nnethercote/servo,peterjoel/servo,saneyuki/servo,pyfisch/servo,KiChjang/servo,notriddle/servo,paulrouget/servo,szeged/servo,KiChjang/servo,larsbergstrom/servo,nnethercote/servo,larsbergstrom/servo,emilio/servo,DominoTree/servo,emilio/servo,peterjoel/servo,nnethercote/servo,larsbergstrom/servo,szeged/servo,notriddle/servo,jimberlage/servo,notriddle/servo,splav/servo,pyfisch/servo,szeged/servo,KiChjang/servo,emilio/servo,emilio/servo,pyfisch/servo,jimberlage/servo,szeged/servo,larsbergstrom/servo,emilio/servo,DominoTree/servo,paulrouget/servo,nnethercote/servo,jimberlage/servo,KiChjang/servo,nnethercote/servo,saneyuki/servo,notriddle/servo,splav/servo,larsbergstrom/servo,jimberlage/servo,larsbergstrom/servo,nnethercote/servo,larsbergstrom/servo,nnethercote/servo,pyfisch/servo,notriddle/servo,emilio/servo,emilio/servo,saneyuki/servo,notriddle/servo,DominoTree/servo,paulrouget/servo,DominoTree/servo,jimberlage/servo,DominoTree/servo,saneyuki/servo,splav/servo,jimberlage/servo,jimberlage/servo,notriddle/servo,szeged/servo