Oilpan: make DOMWindowProperty a GC mixin.

Source/core/editing/DOMSelection.cpp

 /*
  * Copyright (C) 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google 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.
@@ skipping 539 matching lines @@
         return true;
     return node->document() == m_frame->document();
 }
 
 void DOMSelection::addConsoleError(const String& message)
 {
     if (m_treeScope)
         m_treeScope->document().addConsoleMessage(ConsoleMessage::create(JSMessageSource, ErrorMessageLevel, message));
 }
 
+void DOMSelection::trace(Visitor* visitor)
+{
+    visitor->trace(m_treeScope);
+    DOMWindowProperty::trace(visitor);
+}
+
 } // namespace blink