Implement support for closing shadow DOM plugin placeholders.

Source/core/html/shadow/PluginPlaceholderElement.cpp

 // 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.
 
 #include "config.h"
 #include "core/html/shadow/PluginPlaceholderElement.h"
 
 #include "bindings/core/v8/V8PluginPlaceholderElement.h"
 #include "core/dom/Document.h"
 #include "wtf/Assertions.h"
@@ skipping 12 matching lines @@
     ASSERT_UNUSED(success, success);
     return element.release();
 }
 
 void PluginPlaceholderElement::setMessage(const String& message)
 {
     bool success = V8PluginPlaceholderElement::PrivateScript::messageAttributeSetter(document().frame(), this, message);
     ASSERT_UNUSED(success, success);
 }
 
+void PluginPlaceholderElement::setIsCloseable(bool closeable)
+{
+    bool success = V8PluginPlaceholderElement::PrivateScript::closeableAttributeSetter(document().frame(), this, closeable);
+    ASSERT_UNUSED(success, success);
+}
+
 } // namespace blink