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

Source/core/svg/SVGUseElement.cpp

Index: Source/core/svg/SVGUseElement.cpp
diff --git a/Source/core/svg/SVGUseElement.cpp b/Source/core/svg/SVGUseElement.cpp
index 587750b512fef22827b343a36cb51fa3fc20ce31..4fdcc9f7ff1c8b0928366d3a70940ed2cf2e80a4 100644
--- a/Source/core/svg/SVGUseElement.cpp
+++ b/Source/core/svg/SVGUseElement.cpp
@@ -45,7 +45,6 @@ namespace blink {
 
 inline SVGUseElement::SVGUseElement(Document& document)
     : SVGGraphicsElement(SVGNames::useTag, document)
-    , SVGURIReference(this)
     , m_x(SVGAnimatedLength::create(this, SVGNames::xAttr, SVGLength::create(LengthModeWidth), AllowNegativeLengths))
     , m_y(SVGAnimatedLength::create(this, SVGNames::yAttr, SVGLength::create(LengthModeHeight), AllowNegativeLengths))
     , m_width(SVGAnimatedLength::create(this, SVGNames::widthAttr, SVGLength::create(LengthModeWidth), ForbidNegativeLengths))
@@ -54,6 +53,8 @@ inline SVGUseElement::SVGUseElement(Document& document)
     , m_needsShadowTreeRecreation(false)
     , m_svgLoadEventTimer(this, &SVGElement::svgLoadEventTimerFired)
 {
+    SVGURIReference::initialize(this);
+
     ASSERT(hasCustomStyleCallbacks());
 
     addToPropertyMap(m_x);