Auto merge of #17225 - bzbarsky:pseudo-sharing-fixage, r=emilio

Fix revalidation selectors when pseudo-elements are involved.

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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 https://bugzilla.mozilla.org/show_bug.cgi?id=1371112

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