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/EventHandler.webidl

@@ -90,6 +90,11 @@ interface mixin GlobalEventHandlers {
            attribute EventHandler onwaiting;
 };
 
+// https://drafts.csswg.org/css-animations/#interface-globaleventhandlers-idl
+partial interface mixin GlobalEventHandlers {
+           attribute EventHandler onanimationend;
+};
+
 // https://drafts.csswg.org/css-transitions/#interface-globaleventhandlers-idl
 partial interface mixin GlobalEventHandlers {
            attribute EventHandler ontransitionrun;