Add the NonElementParentNode interface

Source/core/dom/NonElementParentNode.idl

Index: Source/core/dom/NonElementParentNode.idl
diff --git a/Source/core/dom/NonElementParentNode.idl b/Source/core/dom/NonElementParentNode.idl
new file mode 100644
index 0000000000000000000000000000000000000000..3f4ef6ec690f5acd3785a0285a29e914d6e539ef
--- /dev/null
+++ b/Source/core/dom/NonElementParentNode.idl
@@ -0,0 +1,12 @@
+// 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.
+
+// https://dom.spec.whatwg.org/#interface-nonelementparentnode
+
+[
+    LegacyTreatAsPartialInterface,
+    NoInterfaceObject, // Always used on target of 'implements'
+] interface NonElementParentNode {
+    [PerWorldBindings] Element? getElementById(DOMString elementId);
+};