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

Source/core/svg/SVGURIReference.cpp

Index: Source/core/svg/SVGURIReference.cpp
diff --git a/Source/core/svg/SVGURIReference.cpp b/Source/core/svg/SVGURIReference.cpp
index e5b48a318aa16533f2c66a2ea4bec3269690f5b6..9dec200fe3c83e960dad54ac870ee4472c6de697 100644
--- a/Source/core/svg/SVGURIReference.cpp
+++ b/Source/core/svg/SVGURIReference.cpp
@@ -27,10 +27,16 @@
 
 namespace blink {
 
-SVGURIReference::SVGURIReference(SVGElement* element)
-    : m_href(SVGAnimatedString::create(element, XLinkNames::hrefAttr, SVGString::create()))
+SVGURIReference::SVGURIReference()
 {
+}
+
+void SVGURIReference::initialize(SVGElement* element)
+{
+    ASSERT(!m_href);
     ASSERT(element);
+
+    m_href = SVGAnimatedString::create(element, XLinkNames::hrefAttr, SVGString::create());
     element->addToPropertyMap(m_href);
 }