Blink Plugins: Remove Shadow DOM Plugin Placeholder

Source/core/plugins/PluginPlaceholder.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 PluginPlaceholder_h
-#define PluginPlaceholder_h
-
-#include "platform/heap/Handle.h"
-
-namespace blink {
-
-class ContainerNode;
-
-class PluginPlaceholder : public WillBeGarbageCollectedMixin {
-public:
-#if !ENABLE(OILPAN)
-    virtual ~PluginPlaceholder() { }
-#endif
-
-    // Loads the plugin placeholder into a container, such as a shadow root.
-    // The placeholder may replace/remove any existing children.
-    virtual void loadIntoContainer(ContainerNode&) = 0;
-};
-
-} // namespace blink
-
-#endif // PluginPlaceholder_h