style: Drop layout.css.isolation.enabled pref.
style: Drop layout.css.isolation.enabled pref.

It was enabled by default in bug 1091885.

Differential Revision: https://phabricator.services.mozilla.com/D15705

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