Add support for [TreatNullAs] extended attribute for reflected attributes

Source/core/html/HTMLElement.idl

Index: Source/core/html/HTMLElement.idl
diff --git a/Source/core/html/HTMLElement.idl b/Source/core/html/HTMLElement.idl
index d4e2f30b312940875b937d6521df7829aea665b5..589c9949150bd39478222b43cf30b6182329fd41 100644
--- a/Source/core/html/HTMLElement.idl
+++ b/Source/core/html/HTMLElement.idl
@@ -21,16 +21,16 @@
 [
     CustomToV8
 ] interface HTMLElement : Element {
-             [Reflect] attribute DOMString title;
-             [Reflect] attribute DOMString lang;
+             [Reflect, TreatNullAs=NullString] attribute DOMString title;
+             [Reflect, TreatNullAs=NullString] attribute DOMString lang;
              attribute boolean             translate;
-             [Reflect] attribute DOMString dir;
+             [Reflect, TreatNullAs=NullString] attribute DOMString dir;
 
              [CustomElementCallbacks] attribute long              tabIndex;
              [CustomElementCallbacks] attribute boolean           draggable;
-             [Reflect] attribute DOMString webkitdropzone;
+             [Reflect, TreatNullAs=NullString] attribute DOMString webkitdropzone;
              [Reflect] attribute boolean hidden;
-             [Reflect] attribute DOMString accessKey;
+             [Reflect, TreatNullAs=NullString] attribute DOMString accessKey;
 
     // Extensions
              [TreatNullAs=NullString, CustomElementCallbacks, PerWorldBindings, ActivityLog=SetterForIsolatedWorlds, SetterRaisesException] attribute DOMString innerHTML;