Add custom element registry

components/script/dom/webidls/CustomElementRegistry.webidl

@@ -0,0 +1,18 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+// https://html.spec.whatwg.org/multipage/#customelementregistry
+[Pref="dom.customelements.enabled"]
+interface CustomElementRegistry {
+  [Throws/*, CEReactions */]
+  void define(DOMString name, Function constructor_, optional ElementDefinitionOptions options);
+
+  any get(DOMString name);
+
+  Promise<void> whenDefined(DOMString name);
+};
+
+dictionary ElementDefinitionOptions {
+  DOMString extends;
+};