Add the CustomElementsRegistry interface behind the flag

third_party/WebKit/Source/core/frame/RemoteDOMWindow.cpp

 // Copyright 2014 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.
 
 #include "core/frame/RemoteDOMWindow.h"
 
 #include "bindings/core/v8/SerializedScriptValue.h"
 #include "core/css/CSSRuleList.h"
 #include "core/css/CSSStyleDeclaration.h"
 #include "core/css/MediaQueryList.h"
@@ skipping 315 matching lines @@
 {
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 void RemoteDOMWindow::cancelIdleCallback(int id)
 {
     ASSERT_NOT_REACHED();
 }
 
+CustomElementsRegistry* RemoteDOMWindow::customElements() const
+{
+    ASSERT_NOT_REACHED();
+    return nullptr;
+}
+
 RemoteDOMWindow::RemoteDOMWindow(RemoteFrame& frame)
     : m_frame(&frame)
 {
 }
 
 void RemoteDOMWindow::frameDetached()
 {
     m_frame = nullptr;
 }
 
 } // namespace blink