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

Source/core/svg/SVGRectTearOff.h

Index: Source/core/svg/SVGRectTearOff.h
diff --git a/Source/core/svg/SVGRectTearOff.h b/Source/core/svg/SVGRectTearOff.h
index 3c04de189501b71c29705083c5ac814b571887ef..a4c7ee51f3b218bf17406302916f1dce50bd7142 100644
--- a/Source/core/svg/SVGRectTearOff.h
+++ b/Source/core/svg/SVGRectTearOff.h
@@ -39,7 +39,7 @@ namespace WebCore {
 
 class SVGRectTearOff : public SVGPropertyTearOff<SVGRect>, public ScriptWrappable {
 public:
-    static PassRefPtr<SVGRectTearOff> create(PassRefPtr<SVGRect> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGRectTearOff> create(PassRefPtr<SVGRect> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGRectTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }
@@ -54,7 +54,7 @@ public:
     float height() { return target()->height(); }
 
 private:
-    SVGRectTearOff(PassRefPtr<SVGRect>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = nullQName());
+    SVGRectTearOff(PassRefPtr<SVGRect>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = QualifiedName::null());
 };
 
 } // namespace WebCore