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

Source/core/svg/SVGTextPathElement.cpp

Index: Source/core/svg/SVGTextPathElement.cpp
diff --git a/Source/core/svg/SVGTextPathElement.cpp b/Source/core/svg/SVGTextPathElement.cpp
index a9990f57196f20fc11ce94c61ac76aaf4301561b..52c5d9aa5594438b4aaefb4eb5f0c4d6be95a27b 100644
--- a/Source/core/svg/SVGTextPathElement.cpp
+++ b/Source/core/svg/SVGTextPathElement.cpp
@@ -49,11 +49,12 @@ template<> const SVGEnumerationStringEntries& getStaticStringEntries<SVGTextPath
 
 inline SVGTextPathElement::SVGTextPathElement(Document& document)
     : SVGTextContentElement(SVGNames::textPathTag, document)
-    , SVGURIReference(this)
     , m_startOffset(SVGAnimatedLength::create(this, SVGNames::startOffsetAttr, SVGLength::create(LengthModeOther), AllowNegativeLengths))
     , m_method(SVGAnimatedEnumeration<SVGTextPathMethodType>::create(this, SVGNames::methodAttr, SVGTextPathMethodAlign))
     , m_spacing(SVGAnimatedEnumeration<SVGTextPathSpacingType>::create(this, SVGNames::spacingAttr, SVGTextPathSpacingExact))
 {
+    SVGURIReference::initialize(this);
+
     addToPropertyMap(m_startOffset);
     addToPropertyMap(m_method);
     addToPropertyMap(m_spacing);