Sync the AnimationEvent and TransitionEvent interfaces with the specs

Source/core/events/AnimationEvent.idl

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+// http://dev.w3.org/csswg/css-animations/#interface-animationevent
+
 [
     Constructor(DOMString type, optional AnimationEventInit eventInitDict),
 ] interface AnimationEvent : Event {
     readonly attribute DOMString animationName;
+    // TODO(philipj): elapsedTime should be float.
     readonly attribute double elapsedTime;
+    // TODO(philipj): readonly attribute DOMString pseudoElement;
 };
-