Oilpan: move MessageChannel to the heap.

Source/core/dom/MessageChannel.cpp

 /*
  * Copyright (C) 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 39 matching lines @@
     , m_port2(MessagePort::create(*context))
 {
     ScriptWrappable::init(this);
     createChannel(m_port1.get(), m_port2.get());
 }
 
 MessageChannel::~MessageChannel()
 {
 }
 
+void MessageChannel::trace(Visitor* visitor)
+{
+    visitor->trace(m_port1);
+    visitor->trace(m_port2);
+}
+
 } // namespace WebCore