Oilpan: reduce visibility of Visitor::trace(const RawPtr<T>&).

Source/heap/Visitor.h

Index: Source/heap/Visitor.h
diff --git a/Source/heap/Visitor.h b/Source/heap/Visitor.h
index 0dc62ab25c9f1f65954d087c797272b87e0e0eff..80623bf29531b79daa213ccdecb17877dff4bd6d 100644
--- a/Source/heap/Visitor.h
+++ b/Source/heap/Visitor.h
@@ -319,17 +319,15 @@ public:
 #endif
     }
 
+#if !ENABLE(OILPAN)
     // Similarly, this trace method is to trace a RawPtrWillBeMember
     // when ENABLE(OILPAN) is not enabled.
     // Remove this once we remove RawPtrWillBeMember.
     template<typename T>
     void trace(const RawPtr<T>&)
     {
-#if ENABLE(OILPAN)
-        // RawPtrs should never be traced.
-        ASSERT_NOT_REACHED();
-#endif
     }
+#endif
 
     // This method marks an object and adds it to the set of objects
     // that should have their trace method called. Since not all