Ensure callbacks transitions are actually followed.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 17c648a536011dd9bdd507947ddd1e6eed220a78..dd2d9b68c1826f6a6ed4931e155ac4ea8e89d82f 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -5390,9 +5390,9 @@ MaybeObject* JSObject::DefineFastAccessor(Name* name,
   LookupResult result(GetIsolate());
   LocalLookup(name, &result);
 
-  if (result.IsFound()
-      && !result.IsPropertyCallbacks()
-      && !result.IsTransition()) return GetHeap()->null_value();
+  if (result.IsFound() && !result.IsPropertyCallbacks()) {
+    return GetHeap()->null_value();
+  }
 
   // Return success if the same accessor with the same attributes already exist.
   AccessorPair* source_accessors = NULL;