style: Add a pref for a simpler -moz- gradient parsing.
style: Add a pref for a simpler -moz- gradient parsing.

This won't reintroduce any of the regressions that were triggered by our
previous attempts to turn off -moz prefixed gradients, and lets us massively
simplify the gradient code, if it sticks.

Differential Revision: https://phabricator.services.mozilla.com/D29346

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