Oilpan: cleanup based on review comments after removal of TreeShared.

Source/core/svg/SVGUseElement.h

Index: Source/core/svg/SVGUseElement.h
diff --git a/Source/core/svg/SVGUseElement.h b/Source/core/svg/SVGUseElement.h
index deb4157f98d5e2c789490b5966f705f196feaaf9..dd29b2bee31070d2aa8b53be4f3e0e004c72c6dc 100644
--- a/Source/core/svg/SVGUseElement.h
+++ b/Source/core/svg/SVGUseElement.h
@@ -55,6 +55,8 @@ public:
 
     virtual void buildPendingResource() OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     SVGUseElement(Document&, bool wasInsertedByParser);
 
@@ -113,7 +115,7 @@ private:
     bool m_wasInsertedByParser;
     bool m_haveFiredLoadEvent;
     bool m_needsShadowTreeRecreation;
-    RefPtr<SVGElementInstance> m_targetElementInstance;
+    RefPtrWillBeMember<SVGElementInstance> m_targetElementInstance;
     ResourcePtr<DocumentResource> m_resource;
     Timer<SVGElement> m_svgLoadEventTimer;
 };