Remove unintentional copies due to RefCell<T>::get
Remove unintentional copies due to RefCell<T>::get

We are calling `get` when we should call `borrow` or `borrow_mut`.  This is
causing bugs with iframes because we append to a copy of the `children` vector
rather than the original.

seanmonstar/servo,KiChjang/servo,echochamber/servo,walac/servo,nerith/servo,cbrewster/servo,bfrohs/servo,rentongzhang/servo,samfoo/servo,dati91/servo,avadacatavra/servo,tschneidereit/servo,caldwell/servo,peterjoel/servo,aweinstock314/servo,DominoTree/servo,codemac/servo,aidanhs/servo,ConnorGBrewster/servo,RenaudParis/servo,peterjoel/servo,szeged/servo,dsandeephegde/servo,jgraham/servo,deokjinkim/servo,nerith/servo,peterjoel/servo,dsandeephegde/servo,saneyuki/servo,RenaudParis/servo,indykish/servo,anthgur/servo,ryancanhelpyou/servo,juzer10/servo,mukilan/servo,nnethercote/servo,zentner-kyle/servo,walac/servo,sadmansk/servo,evilpie/servo,wartman4404/servo,dvberkel/servo,dvberkel/servo,srbhklkrn/SERVOENGINE,caldwell/servo,avadacatavra/servo,eddyb/servo,jimberlage/servo,s142857/servo,nnethercote/servo,karlito40/servo,upsuper/servo,pyecs/servo,wpgallih/servo,dvberkel/servo,anthgur/servo,DominoTree/servo,nerith/servo,mattnenterprise/servo,anthgur/servo,mbrubeck/servo,avadacatavra/servo,tafia/servo,A-deLuna/servo,vks/servo,cbrewster/servo,szeged/servo,akosel/servo,huonw/servo,chotchki/servo,zentner-kyle/servo,froydnj/servo,michaelwu/servo,runarberg/servo,indykish/servo,aweinstock314/servo,emilio/servo,brendandahl/servo,dhananjay92/servo,tempbottle/servo,splav/servo,ConnorGBrewster/servo,g-k/servo,nrc/servo,kindersung/servo,tschneidereit/servo,srbhklkrn/SERVOENGINE,jlegendary/servo,pyecs/servo,aweinstock314/servo,huonw/servo,pgonda/servo,saneyuki/servo,peterjoel/servo,pyecs/servo,mukilan/servo,cbrewster/servo,wartman4404/servo,thiagopnts/servo,mt2d2/servo,wpgallih/servo,zhangjunlei26/servo,juzer10/servo,tempbottle/servo,eddyb/servo,jdramani/servo,pgonda/servo,snf/servo,j3parker/servo,notriddle/servo,rnestler/servo,pgonda/servo,wpgallih/servo,samfoo/servo,cbrewster/servo,Shraddha512/servo,larsbergstrom/servo,thiagopnts/servo,nick-thompson/servo,RenaudParis/servo,notriddle/servo,Adenilson/prototype-viewing-distance,saneyuki/servo,dati91/servo,jgraham/servo,sadmansk/servo,vks/servo,AnthonyBroadCrawford/servo,wpgallih/servo,ryancanhelpyou/servo,GyrosOfWar/servo,evilpie/servo,nerith/servo,boghison/servo,sadmansk/servo,upsuper/servo,kindersung/servo,zhangjunlei26/servo,szeged/servo,GyrosOfWar/servo,DominoTree/servo,samfoo/servo,mbrubeck/servo,dvberkel/servo,Shraddha512/servo,froydnj/servo,paulrouget/servo,rnestler/servo,jimberlage/servo,hyowon/servo,jlegendary/servo,srbhklkrn/SERVOENGINE,karlito40/servo,eddyb/servo,szeged/servo,GreenRecycleBin/servo,nnethercote/servo,upsuper/servo,GreenRecycleBin/servo,walac/servo,nrc/servo,CJ8664/servo,SimonSapin/servo,notriddle/servo,nick-thompson/servo,chotchki/servo,saneyuki/servo,runarberg/servo,indykish/servo,saratang/servo,zentner-kyle/servo,dsandeephegde/servo,SimonSapin/servo,paulrouget/servo,nick-thompson/servo,sadmansk/servo,emilio/servo,mdibaiee/servo,nrc/servo,rentongzhang/servo,larsbergstrom/servo,AnthonyBroadCrawford/servo,mdibaiee/servo,dhananjay92/servo,splav/servo,Shraddha512/servo,indykish/servo,rentongzhang/servo,KiChjang/servo,szeged/servo,evilpie/servo,dmarcos/servo,evilpie/servo,Shraddha512/servo,KiChjang/servo,j3parker/servo,caldwell/servo,aidanhs/servo,fiji-flo/servo,steveklabnik/servo,tafia/servo,nnethercote/servo,CJ8664/servo,s142857/servo,ConnorGBrewster/servo,saneyuki/servo,j3parker/servo,A-deLuna/servo,rixrix/servo,saratang/servo,froydnj/servo,luniv/servo,saneyuki/servo,KiChjang/servo,anthgur/servo,bfrohs/servo,dagnir/servo,j3parker/servo,AnthonyBroadCrawford/servo,A-deLuna/servo,wartman4404/servo,GreenRecycleBin/servo,paulrouget/servo,KiChjang/servo,jlegendary/servo,brendandahl/servo,RenaudParis/servo,mbrubeck/servo,thiagopnts/servo,boghison/servo,echochamber/servo,nnethercote/servo,SimonSapin/servo,chotchki/servo,tschneidereit/servo,caldwell/servo,rentongzhang/servo,michaelwu/servo,dsandeephegde/servo,boghison/servo,kindersung/servo,nnethercote/servo,steveklabnik/servo,jgraham/servo,runarberg/servo,zhangjunlei26/servo,mukilan/servo,jgraham/servo,indykish/servo,dmarcos/servo,jdramani/servo,peterjoel/servo,GreenRecycleBin/servo,bjwbell/servo,pyfisch/servo,A-deLuna/servo,srbhklkrn/SERVOENGINE,mattnenterprise/servo,tempbottle/servo,dagnir/servo,caldwell/servo,thiagopnts/servo,notriddle/servo,meh/servo,nnethercote/servo,aidanhs/servo,shrenikgala/servo,fiji-flo/servo,luniv/servo,paulrouget/servo,evilpie/servo,dati91/servo,pgonda/servo,rixrix/servo,rixrix/servo,walac/servo,WriterOfAlicrow/servo,Adenilson/prototype-viewing-distance,tschneidereit/servo,fiji-flo/servo,boghison/servo,larsbergstrom/servo,WriterOfAlicrow/servo,walac/servo,s142857/servo,michaelwu/servo,evilpie/servo,bfrohs/servo,deokjinkim/servo,codemac/servo,AnthonyBroadCrawford/servo,thiagopnts/servo,brendandahl/servo,wartman4404/servo,hyowon/servo,CJ8664/servo,emilio/servo,youprofit/servo,rentongzhang/servo,dagnir/servo,froydnj/servo,SimonSapin/servo,pyecs/servo,tschneidereit/servo,ryancanhelpyou/servo,splav/servo,jimberlage/servo,GyrosOfWar/servo,g-k/servo,codemac/servo,dsandeephegde/servo,thiagopnts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