IDL: make optional arguments (without default) explicit sometimes

Source/core/events/HashChangeEvent.idl

Index: Source/core/events/HashChangeEvent.idl
diff --git a/Source/core/events/HashChangeEvent.idl b/Source/core/events/HashChangeEvent.idl
index e5101b8d99186dc47978d81a5b0c5457a256b342..b154776f63f2a5c105b1b068f5ac6d5a164b149f 100644
--- a/Source/core/events/HashChangeEvent.idl
+++ b/Source/core/events/HashChangeEvent.idl
@@ -21,11 +21,11 @@
 [
     EventConstructor,
 ] interface HashChangeEvent : Event {
-    void initHashChangeEvent([Default=Undefined] optional DOMString type,
-                             [Default=Undefined] optional boolean canBubble,
-                             [Default=Undefined] optional boolean cancelable,
-                             [Default=Undefined] optional DOMString oldURL,
-                             [Default=Undefined] optional DOMString newURL);
+    void initHashChangeEvent(optional DOMString type,
+                             optional boolean canBubble,
+                             optional boolean cancelable,
+                             optional DOMString oldURL,
+                             optional DOMString newURL);
     [InitializedByEventConstructor] readonly attribute DOMString oldURL;
     [InitializedByEventConstructor] readonly attribute DOMString newURL;
 };