Introduce CustomElementRegistry#whenDefined() method

third_party/WebKit/Source/core/dom/custom/CustomElementsRegistry.idl

 // Copyright 2016 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.
 
 [
     RuntimeEnabled=CustomElementsV1,
 ] interface CustomElementsRegistry {
     [CallWith=ScriptState, CEReactions, CustomElementCallbacks, RaisesException] void define(DOMString name, Function constructor, optional ElementRegistrationOptions options);
     any get(DOMString name);
+    [CallWith=ScriptState,RaisesException] Promise<void> whenDefined(DOMString name);
 };