Have DOMImplementation::document() return reference.

Source/bindings/tests/idls/TestInterfacePython4.idl

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This is for interface extended attributes that interact with another extended
+// attribute, and thus both cannot be tested at once.
+// The more *minor* extended attribute should be put in this file.
+
+[
+    SetWrapperReferenceFromReference=document,  // Conflicts with [SetWrapperReferenceTo], [SetWrapperReferenceFrom] and [Custom=VisitDOMWrapper]
+] interface TestInterfacePython4 {   
+};