Fix iframes flickering on mouse move.
Fix iframes flickering on mouse move.

Fixes #7867 (and probably several other iframe bugs).

When collecting layers for children of a pipeline, pass through the current
subpage pipeline recursively. This prevents descendany layers (such as scroll
layers) from being collected and re-created on the subsequent paint.

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