IDL: make optional arguments (without default) explicit sometimes

Source/core/events/MessageEvent.idl

Index: Source/core/events/MessageEvent.idl
diff --git a/Source/core/events/MessageEvent.idl b/Source/core/events/MessageEvent.idl
index 3f2fadc8025355443b4fc1536e291226611f5a3f..921536bd71b1fca651fe55206c47a09feca77a25 100644
--- a/Source/core/events/MessageEvent.idl
+++ b/Source/core/events/MessageEvent.idl
@@ -36,12 +36,12 @@
     [InitializedByEventConstructor] readonly attribute EventTarget? source; // May be a Window or a MessagePort
     [InitializedByEventConstructor, Custom=Getter] readonly attribute any data;
     [InitializedByEventConstructor] readonly attribute MessagePort[] ports;
-    [Custom, MeasureAs=InitMessageEvent] void initMessageEvent([Default=Undefined] optional DOMString typeArg,
-                                   [Default=Undefined] optional boolean canBubbleArg,
-                                   [Default=Undefined] optional boolean cancelableArg,
-                                   [Default=Undefined] optional any dataArg,
-                                   [Default=Undefined] optional DOMString originArg,
-                                   [Default=Undefined] optional DOMString lastEventIdArg,
-                                   [Default=Undefined] optional Window sourceArg,
-                                   [Default=Undefined] optional MessagePort[] messagePorts);
+    [Custom, MeasureAs=InitMessageEvent] void initMessageEvent(optional DOMString typeArg,
+                                   optional boolean canBubbleArg,
+                                   optional boolean cancelableArg,
+                                   optional any dataArg,
+                                   optional DOMString originArg,
+                                   optional DOMString lastEventIdArg,
+                                   optional Window sourceArg,
+                                   optional MessagePort[] messagePorts);
 };