Implemented XMLSerializer in PrivateScript.

Source/core/xml/XMLSerializer.idl

Index: Source/core/xml/XMLSerializer.idl
diff --git a/Source/core/xml/XMLSerializer.idl b/Source/core/xml/XMLSerializer.idl
index 1a7b6279dba8f46db31cd54994b0786295794a33..2ff981a1ca477d7b1814c91236092b6135dc20bd 100644
--- a/Source/core/xml/XMLSerializer.idl
+++ b/Source/core/xml/XMLSerializer.idl
@@ -20,8 +20,11 @@
 
 [
     WillBeGarbageCollected,
-    Constructor
+    Constructor,
+    ConstructorCallWith=Document
 ] interface XMLSerializer {
-    [RaisesException] DOMString serializeToString([Default=Undefined] optional Node node);
+    [RaisesException, TypeChecking=Interface] DOMString serializeToString(Node node);
+
+    [OnlyExposedToPrivateScript, ImplementedInPrivateScript] DOMString serializeToStringInternal(Node node, optional DOMString xmlDeclaration = "");

[yosin_UTC9, 2014/09/04 01:10:41]

Q: Do we need to make |xmlDeclaration| parameter as optional? C++ code always
pass this parameter. I think it is better to check empty string, null and
|undefined| in JS. I guess major call sites of this function is in JS. If so,
handling of |xmlDeclaration| as optional parameter in JS makes writing JS call
sites easier. In the future, we can use ES6 optional parameter.

 };