Implement support for closing shadow DOM plugin placeholders.

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