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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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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…states

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -6510,6 +6510,12 @@
             "url": "/_mozilla/mozilla/MouseEvent.html"
           }
         ],
+        "mozilla/activation.html": [
+          {
+            "path": "mozilla/activation.html",
+            "url": "/_mozilla/mozilla/activation.html"
+          }
+        ],
         "mozilla/bad_cert_detected.html": [
           {
             "path": "mozilla/bad_cert_detected.html",

tests/wpt/mozilla/meta/mozilla/activation.html.ini

@@ -0,0 +1,3 @@
+[activation.html]
+  type: testharness
+  prefs: [dom.testing.element.activation.enabled:true]