Store HTMLIFrameElement sandbox attr as TokenList
Store HTMLIFrameElement sandbox attr as TokenList

Fixes #3758

Store the sandbox attribute of HTMLIFrameElement as a TokenList
internally. Use .tokens() to iterate over the tokens instead of
splitting on the string value.

The external interface for sandbox remains a DOMString, which will need
to be fixed when DOMSettableTokenList is implemented (#1717).

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ham/servo,runarberg/servo,kindersung/servo,dmarcos/servo,mbrubeck/servo,evilpie/servo,mt2d2/servo,avadacatavra/servo,steveklabnik/servo,rixrix/servo,wpgallih/servo,ruud-v-a/servo,evilpie/servo,chotchki/servo,luniv/servo,AnthonyBroadCrawford/servo,caldwell/servo,upsuper/servo,mbrubeck/servo,canaltinova/servo,thiagopnts/servo,KiChjang/servo,jlegendary/servo,GyrosOfWar/servo,dati91/servo,sadmansk/servo,evilpie/servo,rnestler/servo,samfoo/servo,pyfisch/servo,larsbergstrom/servo,wpgallih/servo,zentner-kyle/servo,zentner-kyle/servo,srbhklkrn/SERVOENGINE,cbrewster/servo,splav/servo,walac/servo,dhananjay92/servo,rixrix/servo,fiji-flo/servo,pyecs/servo,eddyb/servo,nrc/servo,saratang/servo,anthgur/servo,cbrewster/servo,emilio/servo,notriddle/servo,ruud-v-a/servo,ryancanhelpyou/servo,brendandahl/servo,paulrouget/servo,peterjoel/servo,saratang/servo,bjwbell/servo,peterjoel/servo,karlito40/servo,nrc/servo,meh/servo,fiji-flo/servo,rnestler/servo,GreenRecycleBin/servo,echochamber/servo,canaltinova/servo,kindersung/servo,dagnir/servo,chotchki/servo,wpgallih/servo,pyecs/servo,tempbottle/servo,akosel/servo,wartman4404/servo,codemac/servo,nerith/servo,Adenilson/prototype-viewing-distance,SimonSapin/servo,zentner-kyle/servo,indykish/servo,aweinstock314/servo,peterjoel/servo,upsuper/servo,runarberg/servo,mdibaiee/servo,WriterOfAlicrow/servo,eddyb/servo,rixrix/servo,emilio/servo,pgonda/servo,brendandahl/servo