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..7fa995fe4cb713e9c6caf4cbbf4899304c4d1cd8
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+++ b/Source/core/html/shadow/PluginPlaceholderElement.h
@@ -0,0 +1,28 @@
+// 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 {
+public:
+    // Returns a null pointer if the placeholder DOM could not be built.
+    static PassRefPtrWillBeRawPtr<PluginPlaceholderElement> create(Document&);
+
+    void setMessage(const String&);
+
+private:
+    explicit PluginPlaceholderElement(Document&);
+
+    void initializePlaceholderElements();
+};
+
+} // namespace blink
+
+#endif // PluginPlaceholderElement_h