Auto merge of #16873 - bzbarsky:fix-dir-matching, r=emilio

Fix dynamic updates when :dir matching changes in stylo.

This is the servo part of https://bugzilla.mozilla.org/show_bug.cgi?id=1364280

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- [X] `./mach build -d` does not report any errors
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- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1364280
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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because Gecko has lots of tests for this and this is Gecko-only.

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