Auto merge of #7751 - mrobinson:refactor-hit-testing, r=pcwalton
Auto merge of #7751 - mrobinson:refactor-hit-testing, r=pcwalton

Make DisplayList hit testing a method on DisplayList

This will allow us to hit test into DisplayLists that are not directly
contained in StackingContexts, but instead are children of PaintLayers.
It also makes things slightly simpler.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7751)
<!-- Reviewable:end -->

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