Add support for animationend event

This is triggered when an animation finishes. This is a high priority
because it allows us to start rooting nodes with animations in the
script thread.

This doesn't yet cause a lot of tests to pass because they rely on the
existence of `Document.getAnimations()` and the presence of
`animationstart` and animationiteration` events.

components/script/dom/webidls/AnimationEvent.webidl

@@ -0,0 +1,26 @@
+/* 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/.
+ *
+ * The origin of this IDL file is
+ * http://www.w3.org/TR/css3-animations/#animation-events-
+ * http://dev.w3.org/csswg/css3-animations/#animation-events-
+ *
+ * Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C
+ * liability, trademark and document use rules apply.
+ */
+
+[Exposed=Window]
+interface AnimationEvent : Event {
+  constructor(DOMString type, optional AnimationEventInit eventInitDict = {});
+
+  readonly attribute DOMString animationName;
+  readonly attribute float     elapsedTime;
+  readonly attribute DOMString pseudoElement;
+};
+
+dictionary AnimationEventInit : EventInit {
+  DOMString animationName = "";
+  float elapsedTime = 0;
+  DOMString pseudoElement = "";
+};