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

Source/core/svg/SVGFitToViewBox.h

 /*
  * Copyright (C) 2004, 2005, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2010 Rob Buis <buis@kde.org>
  * Copyright (C) 2014 Samsung Electronics. All rights reserved.
  *
  * 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.
  *
@@ skipping 55 matching lines @@
 
     bool hasEmptyViewBox() const { return m_viewBox->currentValue()->isValid() && m_viewBox->currentValue()->value().isEmpty(); }
 
     // JS API
     SVGAnimatedRect* viewBox() const { return m_viewBox.get(); }
     SVGAnimatedPreserveAspectRatio* preserveAspectRatio() const { return m_preserveAspectRatio.get(); }
 
     virtual void trace(Visitor*);
 
 protected:
-    explicit SVGFitToViewBox(SVGElement*, PropertyMapPolicy = PropertyMapPolicyAdd);
+    SVGFitToViewBox();
+
+    void initialize(SVGElement* contextElement, PropertyMapPolicy = PropertyMapPolicyAdd);
+
     void updateViewBox(const FloatRect&);
     void clearViewBox() { m_viewBox = nullptr; }
     void clearPreserveAspectRatio() { m_preserveAspectRatio = nullptr; }
 
 private:
     RefPtrWillBeMember<SVGAnimatedRect> m_viewBox;
     RefPtrWillBeMember<SVGAnimatedPreserveAspectRatio> m_preserveAspectRatio;
 };
 
 } // namespace blink
 
 #endif // SVGFitToViewBox_h