Auto merge of #16637 - bzbarsky:probing-lazy-pseudos, r=emilio
Auto merge of #16637 - bzbarsky:probing-lazy-pseudos, r=emilio

Fix Stylist::lazily_compute_pseudo_element_style to return None when it should.

We have a fast path to return None if we have no rules for the pseudo-element at
all, but we should also return None if we have no _matching_ rules for it for
the given originating element.  This is relied on by consumers like
Servo_ResolvePseudoStyle, which needs to be able to detect the "no styles for
this pseudo-element for this originating element case", to support probing for
whether work for a specific pseudo-element should be done at all.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./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 _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- 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/16637)
<!-- Reviewable:end -->

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