style: Add support for read only SharedRwLocks, which don't need any locking.
style: Add support for read only SharedRwLocks, which don't need any locking.

Differential Revision: https://phabricator.services.mozilla.com/D17185

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