Auto merge of #6657 - Ms2ger:ThreadSafeLayoutNodeChildrenIterator, r=pcwalton
Auto merge of #6657 - Ms2ger:ThreadSafeLayoutNodeChildrenIterator, r=pcwalton

Cleanup ThreadSafeLayoutNodeChildrenIterator.



<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6657)
<!-- Reviewable:end -->

dsandeephegde/servo,GyrosOfWar/servo,KiChjang/servo,szeged/servo,dati91/servo,thiagopnts/servo,walac/servo,nerith/servo,wartman4404/servo,steveklabnik/servo,j3parker/servo,CJ8664/servo,nick-thompson/servo,brendandahl/servo,RenaudParis/servo,pyecs/servo,eddyb/servo,runarberg/servo,avadacatavra/servo,mdibaiee/servo,runarberg/servo,jimberlage/servo,dati91/servo,GyrosOfWar/servo,sadmansk/servo,RenaudParis/servo,brendandahl/servo,saneyuki/servo,tempbottle/servo,dati91/servo,emilio/servo,notriddle/servo,meh/servo,szeged/servo,tafia/servo,huonw/servo,KiChjang/servo,aidanhs/servo,rnestler/servo,g-k/servo,jimberlage/servo,mattnenterprise/servo,WriterOfAlicrow/servo,mbrubeck/servo,g-k/servo,WriterOfAlicrow/servo,dhananjay92/servo,mattnenterprise/servo,walac/servo,pyfisch/servo,huonw/servo,sadmansk/servo,aidanhs/servo,DominoTree/servo,youprofit/servo,runarberg/servo,boghison/servo,pyfisch/servo,akosel/servo,tempbottle/servo,zentner-kyle/servo,AnthonyBroadCrawford/servo,splav/servo,CJ8664/servo,nrc/servo,dsandeephegde/servo,KiChjang/servo,jlegendary/servo,cbrewster/servo,caldwell/servo,dvberkel/servo,avadacatavra/servo,anthgur/servo,mbrubeck/servo,dvberkel/servo,dhananjay92/servo,notriddle/servo,paulrouget/servo,vks/servo,fiji-flo/servo,pyecs/servo,codemac/servo,SimonSapin/servo,wartman4404/servo,saneyuki/servo,sadmansk/servo,thiagopnts/servo,zentner-kyle/servo,rnestler/servo,ConnorGBrewster/servo,saratang/servo,DominoTree/servo,zentner-kyle/servo,nnethercote/servo,jimberlage/servo,dvberkel/servo,AnthonyBroadCrawford/servo,szeged/servo,jimberlage/servo,pyecs/servo,ConnorGBrewster/servo,avadacatavra/servo,peterjoel/servo,caldwell/servo,peterjoel/servo,jimberlage/servo,notriddle/servo,aidanhs/servo,nick-thompson/servo,walac/servo,sadmansk/servo,wartman4404/servo,larsbergstrom/servo,nrc/servo,SimonSapin/servo,meh/servo,nnethercote/servo,s142857/servo,mdibaiee/servo,peterjoel/servo,walac/servo,SimonSapin/servo,vks/servo,aweinstock314/servo,avadacatavra/servo,peterjoel/servo,SimonSapin/servo,brendandahl/servo,saneyuki/servo,caldwell/servo,upsuper/servo,paulrouget/servo,saratang/servo,steveklabnik/servo,rentongzhang/servo,fiji-flo/servo,akosel/servo,eddyb/servo,brendandahl/servo,fiji-flo/servo,codemac/servo,mbrubeck/servo,pyecs/servo,szeged/servo,splav/servo,steveklabnik/servo,notriddle/servo,peterjoel/servo,splav/servo,KiChjang/servo,SimonSapin/servo,akosel/servo,fiji-flo/servo,huonw/servo,boghison/servo,dhananjay92/servo,A-deLuna/servo,boghison/servo,anthgur/servo,mbrubeck/servo,DominoTree/servo,CJ8664/servo,rentongzhang/servo,dsandeephegde/servo,akosel/servo,upsuper/servo,mbrubeck/servo,DominoTree/servo,ConnorGBrewster/servo,dagnir/servo,saratang/servo,upsuper/servo,steveklabnik/servo,dhananjay92/servo,mattnenterprise/servo,paulrouget/servo,jimberlage/servo,notriddle/servo,zentner-kyle/servo,KiChjang/servo,A-deLuna/servo,rentongzhang/servo,dagnir/servo,szeged/servo,sadmansk/servo,j3parker/servo,aweinstock314/servo,wartman4404/servo,caldwell/servo,tafia/servo,AnthonyBroadCrawford/servo,g-k/servo,upsuper/servo,nnethercote/servo,thiagopnts/servo,pyfisch/servo,KiChjang/servo,saneyuki/servo,anthgur/servo,nerith/servo,notriddle/servo,tafia/servo,aidanhs/servo,nnethercote/servo,saneyuki/servo,emilio/servo,kindersung/servo,g-k/servo,nerith/servo,g-k/servo,jlegendary/servo,bjwbell/servo,A-deLuna/servo,youprofit/servo,dagnir/servo,kindersung/servo,dsandeephegde/servo,DominoTree/servo,dati91/servo,saneyuki/servo,nerith/servo,nnethercote/servo,nerith/servo,canaltinova/servo,wartman4404/servo,KiChjang/servo,nrc/servo,j3parker/servo,fiji-flo/servo,kindersung/servo,dhananjay92/servo,nick-thompson/servo,nick-thompson/servo,WriterOfAlicrow/servo,mdibaiee/servo,bjwbell/servo,akosel/servo,AnthonyBroadCrawford/servo,dagnir/servo,KiChjang/servo,peterjoel/servo,canaltinova/servo,dsandeephegde/servo,nrc/servo,zentner-kyle/servo,tafia/servo,jlegendary/servo,bjwbell/servo,upsuper/servo,notriddle/servo,emilio/servo,larsbergstrom/servo,DominoTree/servo,A-deLuna/servo,canaltinova/servo,zentner-kyle/servo,mattnenterprise/servo,aidanhs/servo,jdramani/servo,larsbergstrom/servo,szeged/servo,nnethercote/servo,aidanhs/servo,kindersung/servo,huonw/servo,CJ8664/servo,bjwbell/servo,thiagopnts/servo,brendandahl/servo,KiChjang/servo,pyecs/servo,avadacatavra/servo,nnethercote/servo,RenaudParis/servo,fiji-flo/servo,larsbergstrom/servo,dhananjay92/servo,dagnir/servo,wartman4404/servo,cbrewster/servo,jimberlage/servo,canaltinova/servo,tafia/servo,avadacatavra/servo,dsandeephegde/servo,saneyuki/servo,s142857/servo,splav/servo,aweinstock314/servo,larsbergstrom/servo,paulrouget/servo,s142857/servo,paulrouget/servo,bjwbell/servo,peterjoel/servo,notriddle/servo,nrc/servo,anthgur/servo,tempbottle/servo,boghison/servo,youprofit/servo,runarberg/servo,splav/servo,caldwell/servo,SimonSapin/servo,AnthonyBroadCrawford/servo,bjwbell/servo,youprofit/servo,pyfisch/servo,steveklabnik/servo,szeged/servo,dati91/servo,tafia/servo,wartman4404/servo,mdibaiee/servo,vks/servo,rentongzhang/servo,upsuper/servo,rentongzhang/servo,brendandahl/servo,thiagopnts/servo,eddyb/servo,larsbergstrom/servo,huonw/servo,pyecs/servo,larsbergstrom/servo,aweinstock314/servo,brendandahl/servo,canaltinova/servo,splav/servo,emilio/servo,rnestler/servo,DominoTree/servo,tempbottle/servo,saratang/servo,canaltinova/servo,splav/servo,upsuper/servo,mbrubeck/servo,nnethercote/servo,saratang/servo,j3parker/servo,ConnorGBrewster/servo,dati91/servo,s142857/servo,canaltinova/servo,thiagopnts/servo,thiagopnts/servo,s142857/servo,mattnenterprise/servo,paulrouget/servo,ConnorGBrewster/servo,boghison/servo,eddyb/servo,upsuper/servo,jdramani/servo,codemac/servo,dagnir/servo,AnthonyBroadCrawford/servo,jlegendary/servo,jdramani/servo,dati91/servo,zentner-kyle/servo,dsandeephegde/servo,j3parker/servo,paulrouget/servo,ConnorGBrewster/servo,dagnir/servo,j3parker/servo,mdibaiee/servo,tempbottle/servo,peterjoel/servo,cbrewster/servo,cbrewster/servo,paulrouget/servo,dhananjay92/servo,WriterOfAlicrow/servo,avadacatavra/servo,anthgur/servo,nrc/servo,avadacatavra/servo,cbrewster/servo,akosel/servo,notriddle/servo,tempbottle/servo,runarberg/servo,cbrewster/servo,anthgur/servo,WriterOfAlicrow/servo,sadmansk/servo,canaltinova/servo,cbrewster/servo,A-deLuna/servo,dvberkel/servo,mattnenterprise/servo,szeged/servo,A-deLuna/servo,emilio/servo,GyrosOfWar/servo,meh/servo,eddyb/servo,bjwbell/servo,RenaudParis/servo,tempbottle/servo,ConnorGBrewster/servo,emilio/servo,g-k/servo,pyecs/servo,thiagopnts/servo,caldwell/servo,jimberlage/servo,meh/servo,A-deLuna/servo,codemac/servo,DominoTree/servo,g-k/servo,sadmansk/servo,rnestler/servo,pyfisch/servo,nrc/servo,aidanhs/servo,j3parker/servo,dati91/servo,runarberg/servo,dsandeephegde/servo,rnestler/servo,pyfisch/servo,jdramani/servo,pyfisch/servo,mattnenterprise/servo,emilio/servo,CJ8664/servo,szeged/servo,meh/servo,pyfisch/servo,larsbergstrom/servo,WriterOfAlicrow/servo,nerith/servo,GyrosOfWar/servo,pyfisch/servo,paulrouget/servo,kindersung/servo,codemac/servo,CJ8664/servo,huonw/servo,saratang/servo,saneyuki/servo,larsbergstrom/servo,eddyb/servo,pyfisch/servo,jlegendary/servo,runarberg/servo,kindersung/servo,vks/servo,peterjoel/servo,ConnorGBrewster/servo,peterjoel/servo,walac/servo,anthgur/servo,meh/servo,eddyb/servo,WriterOfAlicrow/servo,mattnenterprise/servo,mbrubeck/servo,saneyuki/servo,jdramani/servo,huonw/servo,GyrosOfWar/servo,jlegendary/servo,fiji-flo/servo,nick-thompson/servo,mdibaiee/servo,dvberkel/servo,paulrouget/servo,rentongzhang/servo,youprofit/servo,steveklabnik/servo,emilio/servo,youprofit/servo,meh/servo,CJ8664/servo,rentongzhang/servo,emilio/servo,dvberkel/servo,GyrosOfWar/servo,SimonSapin/servo,walac/servo,dvberkel/servo,rnestler/servo,GyrosOfWar/servo,steveklabnik/servo,youprofit/servo,rnestler/servo,jimberlage/servo,vks/servo,DominoTree/servo,mdibaiee/servo,nnethercote/servo,RenaudParis/servo,s142857/servo,vks/servo,vks/servo,jdramani/servo,splav/servo,KiChjang/servo,emilio/servo,aweinstock314/servo,sadmansk/servo,larsbergstrom/servo,aweinstock314/servo,saneyuki/servo,s142857/servo,RenaudParis/servo,nick-thompson/servo,walac/servo,szeged/servo,boghison/servo,AnthonyBroadCrawford/servo,anthgur/servo,codemac/servo,CJ8664/servo,rnestler/servo,jlegendary/servo,cbrewster/servo,tafia/servo,codemac/servo,fiji-flo/servo,akosel/servo,splav/servo,notriddle/servo,eddyb/servo,nick-thompson/servo,SimonSapin/servo,caldwell/servo,kindersung/servo,saratang/servo,nnethercote/servo,DominoTree/servo,boghison/servo,jdramani/servo,mbrubeck/servo,splav/servo,RenaudParis/servo,aweinstock314/servo,nerith/servo