IDL: make optional arguments (without default) explicit sometimes

Source/core/css/CSSKeyframesRule.idl

Index: Source/core/css/CSSKeyframesRule.idl
diff --git a/Source/core/css/CSSKeyframesRule.idl b/Source/core/css/CSSKeyframesRule.idl
index d85f8d19f17b2484fc260fad6606ba56b61db509..bc86d141bba3b80fb37f412981d65a9f1a782a8a 100644
--- a/Source/core/css/CSSKeyframesRule.idl
+++ b/Source/core/css/CSSKeyframesRule.idl
@@ -32,7 +32,7 @@ interface CSSKeyframesRule : CSSRule {
     readonly attribute CSSRuleList cssRules;
 
     [ImplementedAs=item, NotEnumerable] getter CSSKeyframeRule (unsigned long index);
-    void insertRule([Default=Undefined] optional DOMString rule);
-    void deleteRule([Default=Undefined] optional DOMString key);
-    CSSKeyframeRule findRule([Default=Undefined] optional DOMString key);
+    void insertRule(optional DOMString rule);
+    void deleteRule(optional DOMString key);
+    CSSKeyframeRule findRule(optional DOMString key);
 };