Oilpan: Ensure that classes with virtual trace methods always have vtables for their left-most base…

Source/core/plugins/DOMPlugin.h

 /*
     Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
 
     This library is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 33 matching lines @@
     String name() const;
     String filename() const;
     String description() const;
 
     unsigned length() const;
 
     PassRefPtrWillBeRawPtr<DOMMimeType> item(unsigned index);
     bool canGetItemsForName(const AtomicString& propertyName);
     PassRefPtrWillBeRawPtr<DOMMimeType> namedItem(const AtomicString& propertyName);
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) { }
 
 private:
     const PluginInfo& pluginInfo() const { return m_pluginData->plugins()[m_index]; }
 
     DOMPlugin(PluginData*, LocalFrame*, unsigned index);
     RefPtr<PluginData> m_pluginData;
     unsigned m_index;
 };
 
 } // namespace blink
 
 #endif // Plugin_h