Move around specificity computation so that we know it by the time we mint the Selector.
Move around specificity computation so that we know it by the time we mint the Selector.

This is important to make the selector immutable, which needs to happen when we stick it in an Arc.

MozReview-Commit-ID: BaMbOEbYC3D

ConnorGBrewster/servo,dsandeephegde/servo,thiagopnts/servo,peterjoel/servo,notriddle/servo,szeged/servo,fiji-flo/servo,CJ8664/servo,jimberlage/servo,emilio/servo,saneyuki/servo,nnethercote/servo,paulrouget/servo,cbrewster/servo,saneyuki/servo,splav/servo,szeged/servo,dati91/servo,cbrewster/servo,nnethercote/servo,dsandeephegde/servo,peterjoel/servo,upsuper/servo,paulrouget/servo,nnethercote/servo,mbrubeck/servo,splav/servo,thiagopnts/servo,larsbergstrom/servo,paulrouget/servo,emilio/servo,rnestler/servo,anthgur/servo,upsuper/servo,eddyb/servo,SimonSapin/servo,anthgur/servo,jimberlage/servo,anthgur/servo,upsuper/servo,notriddle/servo,CJ8664/servo,splav/servo,CJ8664/servo,nnethercote/servo,peterjoel/servo,DominoTree/servo,fiji-flo/servo,mattnenterprise/servo,avadacatavra/servo,DominoTree/servo,larsbergstrom/servo,mattnenterprise/servo,cbrewster/servo,DominoTree/servo,emilio/servo,upsuper/servo,jimberlage/servo,fiji-flo/servo,mattnenterprise/servo,CJ8664/servo,fiji-flo/servo,pyfisch/servo,CJ8664/servo,canaltinova/servo,larsbergstrom/servo,saneyuki/servo,pyfisch/servo,thiagopnts/servo,anthgur/servo,canaltinova/servo,mbrubeck/servo,larsbergstrom/servo,anthgur/servo,fiji-flo/servo,SimonSapin/servo,SimonSapin/servo,pyfisch/servo,peterjoel/servo,notriddle/servo,avadacatavra/servo,szeged/servo,fiji-flo/servo,anthgur/servo,eddyb/servo,notriddle/servo,notriddle/servo,paulrouget/servo,CJ8664/servo,szeged/servo,pyfisch/servo,saneyuki/servo,dati91/servo,pyfisch/servo,DominoTree/servo,splav/servo,paulrouget/servo,rnestler/servo,jimberlage/servo,KiChjang/servo,upsuper/servo,nnethercote/servo,mattnenterprise/servo,rnestler/servo,mbrubeck/servo,dati91/servo,jimberlage/servo,cbrewster/servo,notriddle/servo,canaltinova/servo,notriddle/servo,DominoTree/servo,jimberlage/servo,szeged/servo,jimberlage/servo,peterjoel/servo,DominoTree/servo,saneyuki/servo,sadmansk/servo,larsbergstrom/servo,saneyuki/servo,ConnorGBrewster/servo,sadmansk/servo,rnestler/servo,dati91/servo,sadmansk/servo,splav/servo,mbrubeck/servo,splav/servo,mbrubeck/servo,canaltinova/servo,eddyb/servo,eddyb/servo,splav/servo,KiChjang/servo,nnethercote/servo,eddyb/servo,rnestler/servo,avadacatavra/servo,splav/servo,peterjoel/servo,peterjoel/servo,sadmansk/servo,canaltinova/servo,avadacatavra/servo,mattnenterprise/servo,thiagopnts/servo,upsuper/servo,KiChjang/servo,dsandeephegde/servo,nnethercote/servo,KiChjang/servo,splav/servo,sadmansk/servo,thiagopnts/servo,paulrouget/servo,larsbergstrom/servo,anthgur/servo,fiji-flo/servo,pyfisch/servo,peterjoel/servo,emilio/servo,dsandeephegde/servo,szeged/servo,ConnorGBrewster/servo,saneyuki/servo,fiji-flo/servo,cbrewster/servo,SimonSapin/servo,emilio/servo,paulrouget/servo,mbrubeck/servo,pyfisch/servo,KiChjang/servo,cbrewster/servo,SimonSapin/servo,notriddle/servo,peterjoel/servo,mbrubeck/servo,dati91/servo,KiChjang/servo,paulrouget/servo,canaltinova/servo,sadmansk/servo,mbrubeck/servo,thiagopnts/servo,jimberlage/servo,ConnorGBrewster/servo,avadacatavra/servo,notriddle/servo,nnethercote/servo,dsandeephegde/servo,ConnorGBrewster/servo,thiagopnts/servo,saneyuki/servo,nnethercote/servo,ConnorGBrewster/servo,canaltinova/servo,SimonSapin/servo,emilio/servo,pyfisch/servo,larsbergstrom/servo,larsbergstrom/servo,rnestler/servo,szeged/servo,sadmansk/servo,anthgur/servo,pyfisch/servo,SimonSapin/servo,upsuper/servo,mattnenterprise/servo,ConnorGBrewster/servo,szeged/servo,rnestler/servo,dati91/servo,cbrewster/servo,szeged/servo,dati91/servo,peterjoel/servo,thiagopnts/servo,upsuper/servo,szeged/servo,emilio/servo,splav/servo,eddyb/servo,CJ8664/servo,dsandeephegde/servo,ConnorGBrewster/servo,KiChjang/servo,emilio/servo,KiChjang/servo,DominoTree/servo,eddyb/servo,avadacatavra/servo,KiChjang/servo,sadmansk/servo,dati91/servo,avadacatavra/servo,dsandeephegde/servo,paulrouget/servo,saneyuki/servo,larsbergstrom/servo,emilio/servo,DominoTree/servo,paulrouget/servo,notriddle/servo,canaltinova/servo,DominoTree/servo,DominoTree/servo,emilio/servo,nnethercote/servo,saneyuki/servo,KiChjang/servo,eddyb/servo,larsbergstrom/servo,mattnenterprise/servo,SimonSapin/servo,cbrewster/servo,CJ8664/servo,dsandeephegde/servo,rnestler/servo,jimberlage/servo,avadacatavra/servo,pyfisch/servo,mattnenterprise/servo