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

Source/core/svg/SVGGradientElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) Research In Motion Limited 2010. 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 29 matching lines @@
     if (entries.isEmpty()) {
         entries.append(std::make_pair(SVGSpreadMethodPad, "pad"));
         entries.append(std::make_pair(SVGSpreadMethodReflect, "reflect"));
         entries.append(std::make_pair(SVGSpreadMethodRepeat, "repeat"));
     }
     return entries;
 }
 
 SVGGradientElement::SVGGradientElement(const QualifiedName& tagName, Document& document)
     : SVGElement(tagName, document)
-    , SVGURIReference(this)
     , m_gradientTransform(SVGAnimatedTransformList::create(this, SVGNames::gradientTransformAttr, SVGTransformList::create()))
     , m_spreadMethod(SVGAnimatedEnumeration<SVGSpreadMethodType>::create(this, SVGNames::spreadMethodAttr, SVGSpreadMethodPad))
     , m_gradientUnits(SVGAnimatedEnumeration<SVGUnitTypes::SVGUnitType>::create(this, SVGNames::gradientUnitsAttr, SVGUnitTypes::SVG_UNIT_TYPE_OBJECTBOUNDINGBOX))
 {
+    SVGURIReference::initialize(this);
+
     addToPropertyMap(m_gradientTransform);
     addToPropertyMap(m_spreadMethod);
     addToPropertyMap(m_gradientUnits);
 }
 
 void SVGGradientElement::trace(Visitor* visitor)
 {
     visitor->trace(m_gradientTransform);
     visitor->trace(m_spreadMethod);
     visitor->trace(m_gradientUnits);
@@ skipping 54 matching lines @@
         offset = std::min(std::max(previousOffset, offset), 1.0f);
         previousOffset = offset;
 
         stops.append(Gradient::ColorStop(offset, stop->stopColorIncludingOpacity()));
     }
 
     return stops;
 }
 
 }