Bug 909859. Make the doctype argument of createDocument optional. r=smaug

dom/imptests/webapps/DOMCore/tests/approved/test_interfaces.html

@@ -245,7 +245,7 @@ interface XMLDocument : Document {};
 
 interface DOMImplementation {
   DocumentType createDocumentType(DOMString qualifiedName, DOMString publicId, DOMString systemId);
-  XMLDocument createDocument(DOMString? namespace, [TreatNullAs=EmptyString] DOMString qualifiedName, DocumentType? doctype);
+  XMLDocument createDocument(DOMString? namespace, [TreatNullAs=EmptyString] DOMString qualifiedName, optional DocumentType? doctype = null);
   Document createHTMLDocument(optional DOMString title);
 
   boolean hasFeature(DOMString feature, [TreatNullAs=EmptyString] DOMString version);

dom/webidl/DOMImplementation.webidl

@@ -21,7 +21,7 @@ interface DOMImplementation {
   [Throws]
   Document createDocument(DOMString? namespace,
                           [TreatNullAs=EmptyString] DOMString qualifiedName,
-                          DocumentType? doctype);
+                          optional DocumentType? doctype = null);
   [Throws]
   Document createHTMLDocument(optional DOMString title);
 };