Implement XMLSerializer interface

components/script/dom/webidls/XMLSerializer.webidl

@@ -0,0 +1,13 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+/*
+ * The origin of this IDL file is
+ * https://w3c.github.io/DOM-Parsing/#the-domparser-interface
+ */
+
+[Constructor]
+interface XMLSerializer {
+  [Throws]
+  DOMString serializeToString(Node root);
+};