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

Source/core/svg/SVGMaskElement.cpp

Index: Source/core/svg/SVGMaskElement.cpp
diff --git a/Source/core/svg/SVGMaskElement.cpp b/Source/core/svg/SVGMaskElement.cpp
index 47462002fa618be51db166c04fbb96b538e33287..c26ebfe9c5dcf9a057437a9c935996352eaec80a 100644
--- a/Source/core/svg/SVGMaskElement.cpp
+++ b/Source/core/svg/SVGMaskElement.cpp
@@ -31,7 +31,6 @@ namespace blink {
 
 inline SVGMaskElement::SVGMaskElement(Document& document)
     : SVGElement(SVGNames::maskTag, document)
-    , SVGTests(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))
@@ -39,6 +38,8 @@ inline SVGMaskElement::SVGMaskElement(Document& document)
     , m_maskUnits(SVGAnimatedEnumeration<SVGUnitTypes::SVGUnitType>::create(this, SVGNames::maskUnitsAttr, SVGUnitTypes::SVG_UNIT_TYPE_OBJECTBOUNDINGBOX))
     , m_maskContentUnits(SVGAnimatedEnumeration<SVGUnitTypes::SVGUnitType>::create(this, SVGNames::maskContentUnitsAttr, SVGUnitTypes::SVG_UNIT_TYPE_USERSPACEONUSE))
 {
+    SVGTests::initialize(this);
+
     // Spec: If the x/y attribute is not specified, the effect is as if a value of "-10%" were specified.
     m_x->setDefaultValueAsString("-10%");
     m_y->setDefaultValueAsString("-10%");