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

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 02613ffb62168ff2fce5286458324ac3915ce6b6..e82751ace3fd7baa87055758e7a8f81f36696de7 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -777,12 +777,12 @@ static inline QualifiedName createQualifiedName(const AtomicString& namespaceURI
 {
     AtomicString prefix, localName;
     if (!Document::parseQualifiedName(qualifiedName, prefix, localName, exceptionState))
-        return nullQName();
+        return QualifiedName::null();
 
     QualifiedName qName(prefix, localName, namespaceURI);
     if (!Document::hasValidNamespaceForElements(qName)) {
         exceptionState.throwDOMException(NamespaceError, "The namespace URI provided ('" + namespaceURI + "') is not valid for the qualified name provided ('" + qualifiedName + "').");
-        return nullQName();
+        return QualifiedName::null();
     }
 
     return qName;
@@ -791,7 +791,7 @@ static inline QualifiedName createQualifiedName(const AtomicString& namespaceURI
 PassRefPtrWillBeRawPtr<Element> Document::createElementNS(const AtomicString& namespaceURI, const AtomicString& qualifiedName, ExceptionState& exceptionState)
 {
     QualifiedName qName(createQualifiedName(namespaceURI, qualifiedName, exceptionState));
-    if (qName == nullQName())
+    if (qName == QualifiedName::null())
         return nullptr;
 
     return createElement(qName, false);
@@ -800,7 +800,7 @@ PassRefPtrWillBeRawPtr<Element> Document::createElementNS(const AtomicString& na
 PassRefPtrWillBeRawPtr<Element> Document::createElementNS(const AtomicString& namespaceURI, const AtomicString& qualifiedName, const AtomicString& typeExtension, ExceptionState& exceptionState)
 {
     QualifiedName qName(createQualifiedName(namespaceURI, qualifiedName, exceptionState));
-    if (qName == nullQName())
+    if (qName == QualifiedName::null())
         return nullptr;
 
     RefPtrWillBeRawPtr<Element> element;