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/macros.rs

@@ -442,6 +442,7 @@ macro_rules! global_event_handlers(
     );
     (NoOnload) => (
         event_handler!(abort, GetOnabort, SetOnabort);
+        event_handler!(animationend, GetOnanimationend, SetOnanimationend);
         event_handler!(cancel, GetOncancel, SetOncancel);
         event_handler!(canplay, GetOncanplay, SetOncanplay);
         event_handler!(canplaythrough, GetOncanplaythrough, SetOncanplaythrough);