Auto merge of #18350 - emilio:ship-system-metric-stuff, r=bholley,manishearth
Auto merge of #18350 - emilio:ship-system-metric-stuff, r=bholley,manishearth

stylo: Expose -moz-system-metric and -moz-empty-except-children-with-localname

They don't have the flag in Gecko.

Though I wonder if we could unship them from non-UA sheets.

Bug: 1396048
Reviewed-by: bholley
MozReview-Commit-ID: LGzGDjCZpJC

<!-- 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/18350)
<!-- Reviewable:end -->

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