Add the CustomElementsRegistry interface behind the flag

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

 // 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.
 
 #ifndef RemoteDOMWindow_h
 #define RemoteDOMWindow_h
 
 #include "core/frame/DOMWindow.h"
 #include "core/frame/RemoteFrame.h"
 
@@ skipping 60 matching lines @@
     void resizeBy(int x, int y) const override;
     void resizeTo(int width, int height) const override;
     MediaQueryList* matchMedia(const String&) override;
     CSSStyleDeclaration* getComputedStyle(Element*, const String& pseudoElt) const override;
     CSSRuleList* getMatchedCSSRules(Element*, const String& pseudoElt) const override;
     int requestAnimationFrame(FrameRequestCallback*) override;
     int webkitRequestAnimationFrame(FrameRequestCallback*) override;
     void cancelAnimationFrame(int id) override;
     int requestIdleCallback(IdleRequestCallback*, const IdleRequestOptions&) override;
     void cancelIdleCallback(int id) override;
+    CustomElementsRegistry* customElements() const override;
 
     void frameDetached();
 
 private:
     explicit RemoteDOMWindow(RemoteFrame&);
 
     Member<RemoteFrame> m_frame;
 };
 
 } // namespace blink
 
 #endif // DOMWindow_h