Node.textContent setter should treat undefined the same way as null

Source/core/dom/Node.idl

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 49 matching lines @@
     Node               cloneNode(optional boolean deep);
     [CustomElementCallbacks] void normalize();
 
     // Introduced in DOM Level 2:
     [TreatReturnedNullStringAs=Null, MeasureAs=NodeNamespaceURI] readonly attribute DOMString namespaceURI; // Moved to Element and Attr in DOM4.
     [TreatReturnedNullStringAs=Null, MeasureAs=NodeLocalName] readonly attribute DOMString localName; // Moved to Element and Attr in DOM4.
 
     // Introduced in DOM Level 3:
     [TreatReturnedNullStringAs=Null] readonly attribute DOMString baseURI;
 
-    [TreatReturnedNullStringAs=Null, TreatNullAs=NullString, CustomElementCallbacks] attribute DOMString textContent;
+    [TreatReturnedNullStringAs=Null, TreatNullAs=NullString, TreatUndefinedAs=NullString, CustomElementCallbacks] attribute DOMString textContent;
 
     [MeasureAs=NodeIsSameNode] boolean isSameNode([Default=Undefined] optional Node other); // Removed in DOM4.
     boolean isEqualNode(Node other);
     [TreatReturnedNullStringAs=Null] DOMString lookupPrefix([TreatNullAs=NullString] DOMString namespaceURI);
     boolean isDefaultNamespace([TreatNullAs=NullString] DOMString namespaceURI);
     [TreatReturnedNullStringAs=Null] DOMString lookupNamespaceURI([TreatNullAs=NullString] DOMString prefix);
 
     // DocumentPosition
     const unsigned short      DOCUMENT_POSITION_DISCONNECTED = 0x01;
     const unsigned short      DOCUMENT_POSITION_PRECEDING    = 0x02;
     const unsigned short      DOCUMENT_POSITION_FOLLOWING    = 0x04;
     const unsigned short      DOCUMENT_POSITION_CONTAINS     = 0x08;
     const unsigned short      DOCUMENT_POSITION_CONTAINED_BY = 0x10;
     const unsigned short      DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC = 0x20;
 
     unsigned short compareDocumentPosition(Node other);
 
     // Introduced in DOM4
     boolean contains(Node other);
 
     // IE extensions
     [PerWorldBindings] readonly attribute Element parentElement;
 };