Auto merge of #16607 - canaltinova:moz-transform-var, r=SimonSapin

Fix variable serialization bug on shorthand alias properties

<!-- Please describe your changes on the following line: -->
-moz-transform were having a problem with variable serialization. It wasn't printing transform's value and transform wasn't printing -moz-transform's if their value was a variable. This PR fixes that.
Reduces stylo -moz-transform failures to 2: https://treeherder.mozilla.org/#/jobs?repo=try&revision=3fd3a57873e5e98dfcb173eef4b13822a66fdf6a

---
<!-- 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

<!-- 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/16607)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions