[oilpan]: Avoid refcounting QualifiedName's nullQName when tracing.

Source/core/svg/SVGAngleTearOff.h

Index: Source/core/svg/SVGAngleTearOff.h
diff --git a/Source/core/svg/SVGAngleTearOff.h b/Source/core/svg/SVGAngleTearOff.h
index 8d6d52084b908acc61113efec7c7ae437ba1a538..fd7422b7dfe9fa8a21cfbfb26fe1f00517b30664 100644
--- a/Source/core/svg/SVGAngleTearOff.h
+++ b/Source/core/svg/SVGAngleTearOff.h
@@ -39,7 +39,7 @@ namespace WebCore {
 
 class SVGAngleTearOff FINAL : public SVGPropertyTearOff<SVGAngle>, public ScriptWrappable {
 public:
-    static PassRefPtr<SVGAngleTearOff> create(PassRefPtr<SVGAngle> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGAngleTearOff> create(PassRefPtr<SVGAngle> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGAngleTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }