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

Source/core/dom/Node.idl

Index: Source/core/dom/Node.idl
diff --git a/Source/core/dom/Node.idl b/Source/core/dom/Node.idl
index b8a947fa7416196941f7a09b40eede3e916ef553..a033cd63f0521b39020cab7ec03667169eac5ea9 100644
--- a/Source/core/dom/Node.idl
+++ b/Source/core/dom/Node.idl
@@ -67,7 +67,7 @@
     // 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);