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();
+
+    // FIXME: Oilpan: the construction of this mixin requires heap allocation,
+    // which cannot be safely done with Oilpan until the object that include
+    // this mixin have had its vptr initialized -- so as to be able to
+    // accurately trace the entire object should a GC strike while constructing
+    // the mixin.
+    //
+    // Try to come up with a more natural alternative and solution that doesn't
+    // require hoisting the constructor code for mixins into a separate method
+    // like initialize(). It makes construction of these heap-allocation mixins
+    // safe in the meantime, however.
+    //
+    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