Add more missing closing namespace comments.

third_party/WebKit/Source/core/html/PluginDocument.cpp

 /*
  * Copyright (C) 2006, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 168 matching lines @@
     m_pluginNode = nullptr;
     HTMLDocument::detach(context);
 }
 
 DEFINE_TRACE(PluginDocument)
 {
     visitor->trace(m_pluginNode);
     HTMLDocument::trace(visitor);
 }
 
-}
+} // namespace blink