Add default dictionary value

Author: saschanaz

css-animations-1/Overview.bs

@@ -878,7 +878,7 @@ IDL Definition</h4>
 
 	<pre class="idl">
 		[Exposed=Window,
-		 Constructor(CSSOMString type, optional AnimationEventInit animationEventInitDict)]
+		 Constructor(CSSOMString type, optional AnimationEventInit animationEventInitDict = {})]
 		interface AnimationEvent : Event {
 		  readonly attribute CSSOMString animationName;
 		  readonly attribute double elapsedTime;