Auto merge of #16688 - KuoE0:make-stylo-support-mozmm-unit, r=xidorn

Make stylo support mozmm unit

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This issue is reported at https://bugzilla.mozilla.org/show_bug.cgi?id=1356104. Mozmm is defined at https://developer.mozilla.org/zh-TW/docs/Web/CSS/length.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1356104](https://bugzilla.mozilla.org/show_bug.cgi?id=1356104)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because the tests already exist in Gecko.

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