Add support for [TreatNullAs] extended attribute for reflected attributes

Source/core/html/HTMLFormElement.idl

Index: Source/core/html/HTMLFormElement.idl
diff --git a/Source/core/html/HTMLFormElement.idl b/Source/core/html/HTMLFormElement.idl
index b82e224ace0895cdb3a7b397f89173457a471db7..2ad426cbf584c6d16d9250fcf7334937f9244cb1 100644
--- a/Source/core/html/HTMLFormElement.idl
+++ b/Source/core/html/HTMLFormElement.idl
@@ -19,15 +19,15 @@
  */
 
 interface HTMLFormElement : HTMLElement {
-    [Reflect=accept_charset] attribute DOMString acceptCharset;
-    [Reflect, URL] attribute DOMString action;
-    [Reflect] attribute DOMString autocomplete;
+    [Reflect=accept_charset, TreatNullAs=NullString] attribute DOMString acceptCharset;
+    [Reflect, TreatNullAs=NullString, URL] attribute DOMString action;
+    [Reflect, TreatNullAs=NullString] attribute DOMString autocomplete;
     [TreatNullAs=NullString, CustomElementCallbacks] attribute DOMString enctype;
     [TreatNullAs=NullString, CustomElementCallbacks] attribute DOMString encoding;
     [TreatNullAs=NullString, CustomElementCallbacks] attribute DOMString method;
-    [Reflect] attribute DOMString name;
+    [Reflect, TreatNullAs=NullString] attribute DOMString name;
     [Reflect] attribute boolean noValidate;
-    [Reflect] attribute DOMString target;
+    [Reflect, TreatNullAs=NullString] attribute DOMString target;
 
     readonly attribute HTMLCollection elements;
     readonly attribute long length;