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

Source/core/svg/SVGGradientElement.cpp

Index: Source/core/svg/SVGGradientElement.cpp
diff --git a/Source/core/svg/SVGGradientElement.cpp b/Source/core/svg/SVGGradientElement.cpp
index c4431ebbaca6a18b967655e2e365a7c03915eeaa..99b495e09e5491ae78f3e3d7f36b715fba884f80 100644
--- a/Source/core/svg/SVGGradientElement.cpp
+++ b/Source/core/svg/SVGGradientElement.cpp
@@ -47,11 +47,12 @@ template<> const SVGEnumerationStringEntries& getStaticStringEntries<SVGSpreadMe
 
 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);