Auto merge of #12487 - heycam:supports-ffi, r=emilio

Add FFI glue for Gecko to implement 1-arg CSS.supports() with stylo.

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

This adds an FFI function for Gecko to call to implement the 1-arg version of `CSS.supports()`.  This will be useful for producing an automated analysis of CSS properties we lack support for in geckolib.  Corresponding Gecko bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1287382

r? @emilio

---
<!-- 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
- [ ] `./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 this is a geckolib-only change, and we don't have testing for that yet :(

<!-- 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/12487)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions