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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components/script/dom/element.rs

@@ -2087,6 +2087,27 @@ impl ElementMethods for Element {
         // Step 4.
         self.insert_adjacent(position, fragment.upcast()).map(|_| ())
     }
+
+    // check-tidy: no specs after this line
+    fn EnterFormalActivationState(&self) -> ErrorResult {
+        match self.as_maybe_activatable() {
+            Some(a) => {
+                a.enter_formal_activation_state();
+                return Ok(());
+            },
+            None => return Err(Error::NotSupported)
+        }
+    }
+
+    fn ExitFormalActivationState(&self) -> ErrorResult {
+        match self.as_maybe_activatable() {
+            Some(a) => {
+                a.exit_formal_activation_state();
+                return Ok(());
+            },
+            None => return Err(Error::NotSupported)
+        }
+    }
 }
 
 pub fn fragment_affecting_attributes() -> [Atom; 3] {