Add a custom element to own structure of shadow DOM plugin placeholders.

Source/core/html/HTMLPlugInElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2008, 2009, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 45 matching lines @@
     // Public for FrameView::addWidgetToUpdate()
     bool needsWidgetUpdate() const { return m_needsWidgetUpdate; }
     void setNeedsWidgetUpdate(bool needsWidgetUpdate) { m_needsWidgetUpdate = needsWidgetUpdate; }
     void updateWidget();
 
     bool shouldAccelerate() const;
 
     void requestPluginCreationWithoutRendererIfPossible();
     void createPluginWithoutRenderer();
 
-    bool usePlaceholderContent() { return m_usePlaceholderContent; }
+    bool usePlaceholderContent() const { return m_usePlaceholderContent; }
     void setUsePlaceholderContent(bool);
 
 protected:
     HTMLPlugInElement(const QualifiedName& tagName, Document&, bool createdByParser, PreferPlugInsForImagesOption);
 
     // Node functions:
     virtual void didMoveToNewDocument(Document& oldDocument) OVERRIDE;
 
     // Element functions:
     virtual bool isPresentationAttribute(const QualifiedName&) const OVERRIDE;
@@ skipping 72 matching lines @@
 inline bool isHTMLPlugInElement(const HTMLElement& element)
 {
     return element.isPluginElement();
 }
 
 DEFINE_HTMLELEMENT_TYPE_CASTS_WITH_FUNCTION(HTMLPlugInElement);
 
 } // namespace blink
 
 #endif // HTMLPlugInElement_h