Introduce ConstantSpec::get_value.
Introduce ConstantSpec::get_value.

I'll need it for cross-origin wrappers as well.

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/servo,DominoTree/servo,jdramani/servo,mt2d2/servo,rnestler/servo,pyfisch/servo,zentner-kyle/servo,sadmansk/servo,huonw/servo,pgonda/servo,mukilan/servo,nick-thompson/servo,jimberlage/servo,emilio/servo,bjwbell/servo,canaltinova/servo,jgraham/servo,dsandeephegde/servo,tempbottle/servo,jdramani/servo,codemac/servo,peterjoel/servo,emilio/servo,codemac/servo,vks/servo,rnestler/servo,steveklabnik/servo,GreenRecycleBin/servo,CJ8664/servo,shrenikgala/servo,sadmansk/servo,zhangjunlei26/servo,brendandahl/servo,CJ8664/servo,pgonda/servo,thiagopnts/servo,GyrosOfWar/servo,indykish/servo,chotchki/servo,pyfisch/servo,rixrix/servo,GreenRecycleBin/servo,rentongzhang/servo,walac/servo,codemac/servo,zhangjunlei26/servo,splav/servo,steveklabnik/servo,boghison/servo,nerith/servo,mattnenterprise/servo,jlegendary/servo,larsbergstrom/servo,tempbottle/servo,pyfisch/servo,jlegendary/servo,j3parker/servo,aweinstock314/servo,jimberlage/servo,notriddle/servo,zhangjunlei26/servo,thiagopnts/servo,boghison/servo,splav/servo,akosel/servo,Adenilson/prototype-viewing-distance,GreenRecycleBin/servo,ruud-v-a/servo,tempbottle/servo,g-k/servo,vks/servo,juzer10/servo,tschneidereit/servo,evilpie/servo,mbrubeck/servo,echochamber/servo,indykish/servo,AnthonyBroadCrawford/servo,deokjinkim/servo,peterjoel/servo,larsbergstrom/servo,vks/servo,hyowon/servo,KiChjang/servo,emilio/servo,jimberlage/servo,notriddle/servo,eddyb/servo,cbrewster/servo,cbrewster/servo,dmarcos/servo,codemac/servo,mdibaiee/servo,jdramani/servo,caldwell/servo,AnthonyBroadCrawford/servo,GyrosOfWar/servo,chotchki/servo,bfrohs/servo,michaelwu/servo,tschneidereit/servo,canaltinova/servo,wpgallih/servo,samfoo/servo,saratang/servo,larsbergstrom/servo,bjwbell/servo,ryancanhelpyou/servo,paulrouget/servo,nrc/servo,ConnorGBrewster/servo,larsbergstrom/servo,splav/servo,kindersung/servo,cbrewster/servo,dsandeephegde/servo,nnethercote/servo,shrenikgala/servo,runarberg/servo,upsuper/servo,A-deLuna/servo,pyecs/servo,fiji-flo/servo,larsbergstrom/servo,mbrubeck/servo,pgonda/servo,dhananjay92/servo,aidanhs/servo,hyowon/servo,peterjoel/servo,kindersung/servo