Drop [TreatNullAs=NullString] for Document.designMode

Source/core/dom/Document.idl

Index: Source/core/dom/Document.idl
diff --git a/Source/core/dom/Document.idl b/Source/core/dom/Document.idl
index 29f44df06a75ecaed5313df21c697b1c4a9833d8..5c90a4d1ccfee662bff82b7a3113860d9451bb7d 100644
--- a/Source/core/dom/Document.idl
+++ b/Source/core/dom/Document.idl
@@ -120,8 +120,7 @@ interface Document : Node {
     [ImplementedAs=domWindow] readonly attribute Window? defaultView;
     readonly attribute Element? activeElement;
     boolean hasFocus();
-    // FIXME: designMode should not have [TreatNullAs=NullString].
-    [TreatNullAs=NullString, CustomElementCallbacks, MeasureAs=DocumentDesignMode] attribute DOMString designMode;
+    [CustomElementCallbacks, MeasureAs=DocumentDesignMode] attribute DOMString designMode;
     [CustomElementCallbacks, RaisesException] boolean execCommand(DOMString commandId, optional boolean showUI = false, optional DOMString value = "");
     [RaisesException] boolean queryCommandEnabled(DOMString commandId);
     [RaisesException] boolean queryCommandIndeterm(DOMString commandId);