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

Source/core/svg/SVGAElement.cpp

Index: Source/core/svg/SVGAElement.cpp
diff --git a/Source/core/svg/SVGAElement.cpp b/Source/core/svg/SVGAElement.cpp
index 952b8c76ad502e5ff98ead61cd49e0274f083ee9..12e971eef5b2670b9b178121c9f2f9f322ef5b4b 100644
--- a/Source/core/svg/SVGAElement.cpp
+++ b/Source/core/svg/SVGAElement.cpp
@@ -55,10 +55,11 @@ using namespace HTMLNames;
 
 inline SVGAElement::SVGAElement(Document& document)
     : SVGGraphicsElement(SVGNames::aTag, document)
-    , SVGURIReference(this)
     , m_svgTarget(SVGAnimatedString::create(this, SVGNames::targetAttr, SVGString::create()))
     , m_wasFocusedByMouse(false)
 {
+    SVGURIReference::initialize(this);
+
     addToPropertyMap(m_svgTarget);
 }