Implement support for closing shadow DOM plugin placeholders.

Source/core/html/shadow/PluginPlaceholderElement.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 PluginPlaceholderElement_h
 #define PluginPlaceholderElement_h
 
 #include "core/dom/Element.h"
 #include "core/html/HTMLDivElement.h"
 
 namespace blink {
 
 class PluginPlaceholderElement final : public HTMLDivElement {
     DEFINE_WRAPPERTYPEINFO();
 public:
     static PassRefPtrWillBeRawPtr<PluginPlaceholderElement> create(Document&);
 
     void setMessage(const String&);
+    void setIsCloseable(bool);
 
 private:
     explicit PluginPlaceholderElement(Document&);
 };
 
 } // namespace blink
 
 #endif // PluginPlaceholderElement_h