Blink Plugins: Remove Shadow DOM Plugin Placeholder

public/web/WebPluginPlaceholder.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 WebPluginPlaceholder_h
-#define WebPluginPlaceholder_h
-
-#include "public/platform/WebString.h"
-
-namespace blink {
-
-// Provides content and behavior for a plugin placeholder rendered by Blink.
-class WebPluginPlaceholder {
-public:
-    virtual ~WebPluginPlaceholder() { }
-
-    // Returns a plaintext message to be displayed.
-    virtual WebString message() const { return WebString(); }
-
-    // If set, the placeholder will present UI to permit dismissing it.
-    virtual bool isCloseable() const { return false; }
-};
-
-} // namespace blink
-
-#endif // WebPluginPlaceholder_h