IDL: make optional arguments (without default) explicit sometimes

Source/core/plugins/Plugin.idl

Index: Source/core/plugins/Plugin.idl
diff --git a/Source/core/plugins/Plugin.idl b/Source/core/plugins/Plugin.idl
index 689df505e60c5061d9d790c50eca1f682bb691e3..356107ff4cf020ba25f1e92b2c9e01c6ec31cb05 100644
--- a/Source/core/plugins/Plugin.idl
+++ b/Source/core/plugins/Plugin.idl
@@ -26,8 +26,8 @@
     readonly attribute DOMString filename;
     readonly attribute DOMString description;
     readonly attribute unsigned long length;
-    getter MimeType item([Default=Undefined] optional unsigned long index);
-    MimeType namedItem([Default=Undefined] optional DOMString name);
-    [NotEnumerable, ImplementedAs=namedItem] getter MimeType ([Default=Undefined] optional DOMString name);
+    getter MimeType item(optional unsigned long index);
+    MimeType namedItem(optional DOMString name);
+    [NotEnumerable, ImplementedAs=namedItem] getter MimeType (optional DOMString name);
 };