Don't overwrite transition array map while iterating over the transition tree.

src/objects-visiting-inl.h

Index: src/objects-visiting-inl.h
diff --git a/src/objects-visiting-inl.h b/src/objects-visiting-inl.h
index 31117bb945bb62991245ef676beac33314940167..bea201a6f334b68df21969d1089c23625030a6dd 100644
--- a/src/objects-visiting-inl.h
+++ b/src/objects-visiting-inl.h
@@ -615,12 +615,9 @@ void StaticMarkingVisitor<StaticVisitor>::MarkMapContents(
   // array to prevent visiting it later. Skip recording the transition
   // array slot, since it will be implicitly recorded when the pointer
   // fields of this map are visited.
-  TransitionArray* transitions = map->unchecked_transition_array();
-  if (transitions->IsTransitionArray()) {
+  if (map->HasTransitionArray()) {
+    TransitionArray* transitions = map->transitions();
     MarkTransitionArray(heap, transitions);
-  } else {
-    // Already marked by marking map->GetBackPointer() above.
-    ASSERT(transitions->IsMap() || transitions->IsUndefined());
   }
 
   // Since descriptor arrays are potentially shared, ensure that only the