Make initEvent()/initCustomEvent() arguments optional except the first

This aligns the standard with WebKit and gets it closer to Blink.

Tests: web-platform-tests/wpt#5043.

Fixes #387.

dom.bs

@@ -524,7 +524,7 @@ interface Event {
   [Unforgeable] readonly attribute boolean isTrusted;
   readonly attribute DOMTimeStamp timeStamp;
 
-  void initEvent(DOMString type, boolean bubbles, boolean cancelable); // historical
+  void initEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false); // historical
 };
 
 dictionary EventInit {
@@ -817,7 +817,7 @@ incapable of setting {{Event/composed}}. It has to be supported for legacy conte
 interface CustomEvent : Event {
   readonly attribute any detail;
 
-  void initCustomEvent(DOMString type, boolean bubbles, boolean cancelable, any detail);
+  void initCustomEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false, optional any detail = null);
 };
 
 dictionary CustomEventInit : EventInit {