stylo: Bug 1332633 - Implement ComputeDistance trait.
These patches implement ComputeDistance trait, so we can use it for SMIL animation and for testing test_transitions_per_property.html, which uses nsDOMWindowUtils::ComputeAnimationDistance() to get the the distance between two AnimationValues.
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix Bug 1332633.
- [X] These changes do not require tests because we have test cases in Gecko. (If Servo wants to use this feature, we can add more tests in Servo in the future.)
<!-- 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/16554)
<!-- Reviewable:end -->
350d9c6c47
