It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
CSSOM: Make Stylesheet fields have their own synchronization
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r? @upsuper
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And make empty list the default value of MediaList.
This commit also removes Option wrapper of Stylesheet.media because
a stylesheet should always have an associated media query list.
Support -moz-binding in geckolib
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- [ ] These changes fix #__ (github issue number if applicable).
Either:
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because changes target stylo
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