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

Source/core/svg/SVGNumberTearOff.h

Index: Source/core/svg/SVGNumberTearOff.h
diff --git a/Source/core/svg/SVGNumberTearOff.h b/Source/core/svg/SVGNumberTearOff.h
index d501835089f6789e67b9ddd438915b51828511ba..8abe917f0ce1b204ee1b5b4e4a6283e86f0080c6 100644
--- a/Source/core/svg/SVGNumberTearOff.h
+++ b/Source/core/svg/SVGNumberTearOff.h
@@ -41,7 +41,7 @@ class SVGMatrix;
 
 class SVGNumberTearOff : public SVGPropertyTearOff<SVGNumber>, public ScriptWrappable {
 public:
-    static PassRefPtr<SVGNumberTearOff> create(PassRefPtr<SVGNumber> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGNumberTearOff> create(PassRefPtr<SVGNumber> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGNumberTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }
@@ -50,7 +50,7 @@ public:
     float value() { return target()->value(); }
 
 protected:
-    SVGNumberTearOff(PassRefPtr<SVGNumber>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = nullQName());
+    SVGNumberTearOff(PassRefPtr<SVGNumber>, SVGElement* contextElement, PropertyIsAnimValType, const QualifiedName& attributeName = QualifiedName::null());
 };
 
 } // namespace WebCore