Make [TreatNullAs=NullString] DOMString arguments nullable instead

Source/core/css/CSSStyleDeclaration.idl

Index: Source/core/css/CSSStyleDeclaration.idl
diff --git a/Source/core/css/CSSStyleDeclaration.idl b/Source/core/css/CSSStyleDeclaration.idl
index 0073e9e49801d9096d9c035c9a6b31f11489a11d..45127b629cb599448ae07106cd61b4ec4843cf5c 100644
--- a/Source/core/css/CSSStyleDeclaration.idl
+++ b/Source/core/css/CSSStyleDeclaration.idl
@@ -29,13 +29,13 @@
     [RaisesException] DOMString?          removeProperty([Default=Undefined] optional DOMString propertyName);
     DOMString?          getPropertyPriority([Default=Undefined] optional DOMString propertyName);
     [RaisesException] void setProperty([Default=Undefined] optional DOMString propertyName,
-                                       [TreatNullAs=NullString,Default=Undefined] optional DOMString value,
-                                       [TreatNullAs=NullString] optional DOMString priority = null);
+                                       [Default=Undefined] optional DOMString? value,
+                                       optional DOMString? priority = null);
 
     readonly attribute unsigned long    length;
     getter DOMString          item([Default=Undefined] optional unsigned long index);
     [Custom=PropertyGetter|PropertyEnumerator|PropertyQuery] getter (DOMString or float) (DOMString name);
-    [Custom] setter void (DOMString propertyName, [TreatNullAs=NullString] DOMString propertyValue);
+    [Custom] setter void (DOMString propertyName, DOMString? propertyValue);
     readonly attribute CSSRule          parentRule;
 
     // Deprecated as of 2003: http://lists.w3.org/Archives/Public/www-style/2003Oct/0347.html