Auto merge of #14857 - upsuper:moz-top-layer, r=emilio
Auto merge of #14857 - upsuper:moz-top-layer, r=emilio

Add support for -moz-top-layer

<!-- Please describe your changes on the following line: -->
r? @Manishearth

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14857)
<!-- Reviewable:end -->

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