Initialize GC mixin bases only when leftmost vtable has been initialized.

Source/core/svg/SVGURIReference.h

 /*
  * Copyright (C) 2004, 2005, 2008, 2009 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005 Rob Buis <buis@kde.org>
  *
  * 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,
@@ skipping 40 matching lines @@
     }
 
     const String& hrefString() const { return m_href->currentValue()->value(); }
 
     // JS API
     SVGAnimatedString* href() const { return m_href.get(); }
 
     virtual void trace(Visitor*);
 
 protected:
-    explicit SVGURIReference(SVGElement*);
+    SVGURIReference();
+
+    void initialize(SVGElement*);
 
 private:
     RefPtrWillBeMember<SVGAnimatedString> m_href;
 };
 
 } // namespace blink
 
 #endif // SVGURIReference_h