Auto merge of #18400 - nnethercote:measure-selectors, r=heycam

Measure selectors.

This patch adds measurement of Selectors within StyleRule. This requires
exposing the pointer within ThinArc.

The patch also adds measurement of the several CssRule variants, in order to
measure nested CssRules (and PropertyDeclarationBlocks) within them:
DocumentRule, MediaRule, PageRule, SupportsRule.

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

r? @heycam

---
<!-- 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
- [X] These changes do not require tests because testing is in Gecko.

<!-- 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/18400)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions