Auto merge of #16934 - bzbarsky:check-snapshot-id-class-flags, r=emilio

Check snapshot id/class flags before looking for those attributes.

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Most elements don't have classes or IDs.

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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=1365659

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they are just performance optimization

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