style: Remove now-unused empty Variables struct.
<!-- Please describe your changes on the following line: -->
The `Servo_GetEmptyVariables` FFI function no longer has any callers from Gecko, so we can remove it and the code that creates/frees that struct.
---
<!-- 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/17901)
<!-- Reviewable:end -->
Add message suffixes to stylo error reports.
This is the Servo-side part of https://bugzilla.mozilla.org/show_bug.cgi?id=1381137. This allows us to add the "Declaration dropped" or "Skipped to next declaration" messages after relevant CSS errors that are reported.
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
<!-- 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/17859)
<!-- Reviewable:end -->
It makes just no sense, and I'd rather not complicate the logic in the traversal
implementations, seems easy to just handle it here.
MozReview-Commit-ID: 3PnT2Jyta8g
Store COW strings in CSS parser errors when possible.
This should be a straightforward performance improvement in pages with lots of CSS property values that the parser does not understand.
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because there's no functional difference.
<!-- 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/17787)
<!-- Reviewable:end -->
Replace all uses of the style::stylearc alias with servo_arc.
The `stylearc` alias is left there temporarilly and will be removed completely in a later commit/PR where also `components/style/gecko/generated/structs_{debug|release}.rs` are re-generated (they still use the old alias).
---
<!-- 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
- [X] These changes fix#17768 (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because no new features / only refactoring
<!-- 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/17775)
<!-- Reviewable:end -->
The alias is left there temporarilly and will be removed completely in a later commit where
also components/style/gecko/generated/structs_{debug|release}.rs are re-generated (they still
use the old alias).
This patch also removes the duplication of style contexts during the
restyle, because otherwise pointer equality of ServoComputedValues stops
holding (and we assert on that in a few places)
