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

Source/core/svg/SVGMarkerElement.cpp

Index: Source/core/svg/SVGMarkerElement.cpp
diff --git a/Source/core/svg/SVGMarkerElement.cpp b/Source/core/svg/SVGMarkerElement.cpp
index 5f9cf9df59c462fdbc9edaceba3117f829658601..8262d8904b8478aa1d3bc9248b09eed5969d8f0e 100644
--- a/Source/core/svg/SVGMarkerElement.cpp
+++ b/Source/core/svg/SVGMarkerElement.cpp
@@ -42,7 +42,6 @@ template<> const SVGEnumerationStringEntries& getStaticStringEntries<SVGMarkerUn
 
 inline SVGMarkerElement::SVGMarkerElement(Document& document)
     : SVGElement(SVGNames::markerTag, document)
-    , SVGFitToViewBox(this)
     , m_refX(SVGAnimatedLength::create(this, SVGNames::refXAttr, SVGLength::create(LengthModeWidth), AllowNegativeLengths))
     , m_refY(SVGAnimatedLength::create(this, SVGNames::refYAttr, SVGLength::create(LengthModeHeight), AllowNegativeLengths))
     , m_markerWidth(SVGAnimatedLength::create(this, SVGNames::markerWidthAttr, SVGLength::create(LengthModeWidth), ForbidNegativeLengths))
@@ -50,6 +49,8 @@ inline SVGMarkerElement::SVGMarkerElement(Document& document)
     , m_orientAngle(SVGAnimatedAngle::create(this))
     , m_markerUnits(SVGAnimatedEnumeration<SVGMarkerUnitsType>::create(this, SVGNames::markerUnitsAttr, SVGMarkerUnitsStrokeWidth))
 {
+    SVGFitToViewBox::initialize(this);
+
     // Spec: If the markerWidth/markerHeight attribute is not specified, the effect is as if a value of "3" were specified.
     m_markerWidth->setDefaultValueAsString("3");
     m_markerHeight->setDefaultValueAsString("3");