Add dictionary default value (#4232)

Required after whatwg/webidl#750.

Author: saschanaz

css-transitions-1/Overview.bs

@@ -1086,7 +1086,7 @@ associated with transitions.
 
 <pre class="idl">
   [Exposed=Window,
-   Constructor(CSSOMString type, optional TransitionEventInit transitionEventInitDict)]
+   Constructor(CSSOMString type, optional TransitionEventInit transitionEventInitDict = {})]
   interface TransitionEvent : Event {
     readonly attribute CSSOMString propertyName;
     readonly attribute double elapsedTime;