Auto merge of #17709 - paulrouget:new_pipeline_before_add_pending_changes, r=cbrewster
Auto merge of #17709 - paulrouget:new_pipeline_before_add_pending_changes, r=cbrewster

always call new_pipeline() before add_pending_changes()

There are some changes we want to make in `add_pending_changes` which require pipelines to be registered in self.pipelines.

---
<!-- 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 _____

<!-- 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/17709)
<!-- Reviewable:end -->

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