Add the NonElementParentNode interface

Source/core/dom/NonElementParentNode.h

+// Copyright 2015 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.
+
+#ifndef NonElementParentNode_h
+#define NonElementParentNode_h
+
+#include "core/dom/Document.h"
+#include "core/dom/DocumentFragment.h"
+
+namespace blink {
+
+class NonElementParentNode {
+public:
+    static Element* getElementById(Document& document, const AtomicString& id)
+    {
+        return document.getElementById(id);
+    }
+
+    static Element* getElementById(DocumentFragment& fragment, const AtomicString& id)
+    {
+        return fragment.getElementById(id);
+    }
+};
+
+} // namespace blink
+
+#endif // NonElementParentNode_h