Enable WTF::HashSet<OwnPtr<> >

Source/wtf/HashTraits.h

Index: Source/wtf/HashTraits.h
diff --git a/Source/wtf/HashTraits.h b/Source/wtf/HashTraits.h
index fdf03db06934f77a42f57d1de4787f7c968ace22..eba0bd69d6ecba8b9e39d8a65269c2fc68284118 100644
--- a/Source/wtf/HashTraits.h
+++ b/Source/wtf/HashTraits.h
@@ -144,6 +144,11 @@ namespace WTF {
 
         static EmptyValueType emptyValue() { return nullptr; }
 
+        static const bool hasIsEmptyValueFunction = true;
+        static bool isEmptyValue(const OwnPtr<P>& value) { return !value; }
+
+        typedef typename OwnPtr<P>::PtrType PeekInType;
+
         typedef PassOwnPtr<P> PassInType;
         static void store(PassOwnPtr<P> value, OwnPtr<P>& storage) { storage = value; }