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

Source/core/svg/SVGFilterElement.cpp

Index: Source/core/svg/SVGFilterElement.cpp
diff --git a/Source/core/svg/SVGFilterElement.cpp b/Source/core/svg/SVGFilterElement.cpp
index 115bfb3fabe7d34aaea8d47a47ce64b60eb6dea9..118d8f89f7ce84f865602e689b7843ac96b320c7 100644
--- a/Source/core/svg/SVGFilterElement.cpp
+++ b/Source/core/svg/SVGFilterElement.cpp
@@ -33,7 +33,6 @@ namespace blink {
 
 inline SVGFilterElement::SVGFilterElement(Document& document)
     : SVGElement(SVGNames::filterTag, 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))
@@ -42,6 +41,8 @@ inline SVGFilterElement::SVGFilterElement(Document& document)
     , m_primitiveUnits(SVGAnimatedEnumeration<SVGUnitTypes::SVGUnitType>::create(this, SVGNames::primitiveUnitsAttr, SVGUnitTypes::SVG_UNIT_TYPE_USERSPACEONUSE))
     , m_filterRes(SVGAnimatedIntegerOptionalInteger::create(this, SVGNames::filterResAttr))
 {
+    SVGURIReference::initialize(this);
+
     // Spec: If the x/y attribute is not specified, the effect is as if a value of "-10%" were specified.
     // Spec: If the width/height attribute is not specified, the effect is as if a value of "120%" were specified.
     m_x->setDefaultValueAsString("-10%");