Move creation of plugin placeholder DOM from JS to C++.

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/HTMLButtonElement.h"
 #include "core/html/HTMLDivElement.h"
+#include "wtf/WeakPtr.h"
 
 namespace blink {
 
 class PluginPlaceholderElement final : public HTMLDivElement {
     DEFINE_WRAPPERTYPEINFO();
 public:
-    static PassRefPtrWillBeRawPtr<PluginPlaceholderElement> create(Document&);
+    static PassRefPtrWillBeRawPtr<PluginPlaceholderElement> create(Document& document)
+    {
+        RefPtrWillBeRawPtr<PluginPlaceholderElement> element = adoptRefWillBeNoop(new PluginPlaceholderElement(document));
+        element->initialize();
+        return element.release();
+    }
+
+    ~PluginPlaceholderElement() override;
+    void trace(Visitor*) override;
 
     void setMessage(const String&);
     void setIsCloseable(bool);
 
 private:
     explicit PluginPlaceholderElement(Document&);
+
+    void initialize();
+    PassRefPtr<EventListener> createSimpleEventListener(void(PluginPlaceholderElement::*)());
+
+    void onCloseButtonClick();
+
+    WeakPtrFactory<PluginPlaceholderElement> m_weakPtrFactory;
+    RefPtrWillBeMember<HTMLDivElement> m_messageElement;
+    RefPtrWillBeMember<HTMLButtonElement> m_closeButton;
 };
 
 } // namespace blink
 
 #endif // PluginPlaceholderElement_h