Blink Plugins: Remove Shadow DOM Plugin Placeholder

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/CoreExport.h"
-#include "core/dom/Element.h"
-#include "core/html/HTMLDivElement.h"
-
-namespace blink {
-
-class CORE_EXPORT 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