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

Source/core/html/shadow/PluginPlaceholderElement.h

Index: Source/core/html/shadow/PluginPlaceholderElement.h
diff --git a/Source/core/html/shadow/PluginPlaceholderElement.h b/Source/core/html/shadow/PluginPlaceholderElement.h
index f6fb690fa0a51c2d9064125a9cd4743df2cdf95f..176362b9cbbf671d0c3e4690c8ca289bb2dfff8c 100644
--- a/Source/core/html/shadow/PluginPlaceholderElement.h
+++ b/Source/core/html/shadow/PluginPlaceholderElement.h
@@ -6,20 +6,39 @@
 #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