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

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
new file mode 100644
index 0000000000000000000000000000000000000000..8580881b9cb4bf886a97eb2720a1929a37cbf996
--- /dev/null
+++ b/Source/core/html/shadow/PluginPlaceholderElement.h
@@ -0,0 +1,26 @@
+// 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&);
+
+private:
+    explicit PluginPlaceholderElement(Document&);
+};
+
+} // namespace blink
+
+#endif // PluginPlaceholderElement_h