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

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

<!-- Please describe your changes on the following line: -->
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?

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

…states

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12703)
<!-- Reviewable:end -->

2 files changed, 16 insertions, 0 deletions

diff --git a/components/script/dom/webidls/ActivatableElement.webidl b/components/script/dom/webidls/ActivatableElement.webidl
new file mode 100644
index 00000000000..2b69baff303
--- /dev/null
+++ b/components/script/dom/webidls/ActivatableElement.webidl
@@ -0,0 +1,15 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Interface for testing element activation
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+[Exposed=(Window,Worker), NoInterfaceObject]
+interface ActivatableElement {
+  [Throws, Pref="dom.testing.element.activation.enabled"]
+  void enterFormalActivationState();
+
+  [Throws, Pref="dom.testing.element.activation.enabled"]
+  void exitFormalActivationState();
+};
diff --git a/components/script/dom/webidls/Element.webidl b/components/script/dom/webidls/Element.webidl
index b0ba81d1816..295d76cd1f4 100644
--- a/components/script/dom/webidls/Element.webidl
+++ b/components/script/dom/webidls/Element.webidl
@@ -122,3 +122,4 @@ partial interface Element {
 Element implements ChildNode;
 Element implements NonDocumentTypeChildNode;
 Element implements ParentNode;
+Element implements ActivatableElement;