Add support for [TreatNullAs] extended attribute for reflected attributes

Source/core/html/HTMLButtonElement.idl

Index: Source/core/html/HTMLButtonElement.idl
diff --git a/Source/core/html/HTMLButtonElement.idl b/Source/core/html/HTMLButtonElement.idl
index eb3832fea2ecf8c786d52a01e3316ff4da92092c..e1a6b026d74d2eee4917977bfd05b9ba450979c2 100644
--- a/Source/core/html/HTMLButtonElement.idl
+++ b/Source/core/html/HTMLButtonElement.idl
@@ -22,14 +22,14 @@ interface HTMLButtonElement : HTMLElement {
     [Reflect] attribute boolean autofocus;
     [Reflect] attribute boolean disabled;
     readonly attribute HTMLFormElement form;
-    [Reflect, URL] attribute DOMString formAction;
+    [Reflect, TreatNullAs=NullString, URL] attribute DOMString formAction;
     [TreatNullAs=NullString] attribute DOMString formEnctype;
     [TreatNullAs=NullString] attribute DOMString formMethod;
     [Reflect] attribute boolean formNoValidate;
-    [Reflect] attribute DOMString formTarget;
-    [Reflect] attribute DOMString name;
+    [Reflect, TreatNullAs=NullString] attribute DOMString formTarget;
+    [Reflect, TreatNullAs=NullString] attribute DOMString name;
     [TreatNullAs=NullString] attribute DOMString type;
-    [Reflect] attribute DOMString value;
+    [Reflect, TreatNullAs=NullString] attribute DOMString value;
 
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;