IDL: make optional arguments (without default) explicit sometimes

Source/core/events/CompositionEvent.idl

Index: Source/core/events/CompositionEvent.idl
diff --git a/Source/core/events/CompositionEvent.idl b/Source/core/events/CompositionEvent.idl
index 8b1d740fd687d41e8298b8a90250b04ecdef8c24..b37075da422094c9d71ae64843a52d4485ade3b8 100644
--- a/Source/core/events/CompositionEvent.idl
+++ b/Source/core/events/CompositionEvent.idl
@@ -33,10 +33,10 @@
     [RuntimeEnabled=IMEAPI] readonly attribute long activeSegmentEnd;
     [RuntimeEnabled=IMEAPI] sequence<unsigned long> getSegments();
 
-    void initCompositionEvent([Default=Undefined] optional DOMString typeArg,
-                              [Default=Undefined] optional boolean canBubbleArg,
-                              [Default=Undefined] optional boolean cancelableArg,
-                              [Default=Undefined] optional Window viewArg,
-                              [Default=Undefined] optional DOMString dataArg);
+    void initCompositionEvent(optional DOMString typeArg,
+                              optional boolean canBubbleArg,
+                              optional boolean cancelableArg,
+                              optional Window viewArg,
+                              optional DOMString dataArg);
 
 };