IDL: make optional arguments (without default) explicit sometimes

Source/core/events/MouseEvent.idl

Index: Source/core/events/MouseEvent.idl
diff --git a/Source/core/events/MouseEvent.idl b/Source/core/events/MouseEvent.idl
index feda914c789cd6cf7c5d304f79033e9cfe433d9e..da8d366453f4f34e915d56ec622c7cf527c50ea1 100644
--- a/Source/core/events/MouseEvent.idl
+++ b/Source/core/events/MouseEvent.idl
@@ -35,21 +35,21 @@
     [MeasureAs=PrefixedMouseEventMovementX, ImplementedAs=movementX] readonly attribute long webkitMovementX;
     [MeasureAs=PrefixedMouseEventMovementY, ImplementedAs=movementY] readonly attribute long webkitMovementY;
 
-     void initMouseEvent([Default=Undefined] optional DOMString type,
-                                       [Default=Undefined] optional boolean canBubble,
-                                       [Default=Undefined] optional boolean cancelable,
-                                       [Default=Undefined] optional Window view,
-                                       [Default=Undefined] optional long detail,
-                                       [Default=Undefined] optional long screenX,
-                                       [Default=Undefined] optional long screenY,
-                                       [Default=Undefined] optional long clientX,
-                                       [Default=Undefined] optional long clientY,
-                                       [Default=Undefined] optional boolean ctrlKey,
-                                       [Default=Undefined] optional boolean altKey,
-                                       [Default=Undefined] optional boolean shiftKey,
-                                       [Default=Undefined] optional boolean metaKey,
-                                       [Default=Undefined] optional unsigned short button,
-                                       [Default=Undefined] optional EventTarget relatedTarget);
+     void initMouseEvent(optional DOMString type,
+                                       optional boolean canBubble,
+                                       optional boolean cancelable,
+                                       optional Window view,
+                                       optional long detail,
+                                       optional long screenX,
+                                       optional long screenY,
+                                       optional long clientX,
+                                       optional long clientY,
+                                       optional boolean ctrlKey,
+                                       optional boolean altKey,
+                                       optional boolean shiftKey,
+                                       optional boolean metaKey,
+                                       optional unsigned short button,
+                                       optional EventTarget relatedTarget);
 
     // Non-standard
     [MeasureAs=MouseEventOffsetX] readonly attribute long offsetX;