Auto merge of #11401 - heycam:moz-box, r=mbrubeck

Support display:-moz-(inline-)box in geckolib.

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 --faster` 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 they are geckolib-only changes

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

r? @mbrubeck

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components/style/properties/longhand/box.mako.rs

@@ -19,6 +19,8 @@
             list-item flex
             none
         """.split()
+        if product == "gecko":
+            values += "-moz-box -moz-inline-box".split()
         experimental_values = set("flex".split())
     %>
     pub use self::computed_value::T as SpecifiedValue;