Auto merge of #12703 - sjmelia:12343_test_activation, r=Manishearth

Fix Issue 12343: Add pref-gated methods and test for activatable element

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Add methods to activatable elements; gated by a preference, for use in servo-specific WPT tests.

I cargo-culted over the "redundant check" from #12322, I don't really understand the possible exploit.

Obvious flaw is the copypasta for each element. I understand that the webidl codegen will produce a trait for ActivatableElement, but I can't see how I can provide a default implementation for this? I guess otherwise could be an improvement to have each element just delegate to an Enter.../Exit... method on the Activatable trait?

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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 #12343 (github issue number if applicable).

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