Fix map modification in transition tree traversal.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index cc70bfa8e53a1e96ade1dbf2f6f0c0b17da4a017..3b236d2059153c170bc152c89d64c425a1d58419 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1239,6 +1239,12 @@ void HeapObject::set_map(Map* value) {
 }
 
 
+// Unsafe accessor omitting write barrier.
+void HeapObject::set_map_unsafe(Map* value) {
+  set_map_word(MapWord::FromMap(value));
+}
+
+
 MapWord HeapObject::map_word() {
   return MapWord(reinterpret_cast<uintptr_t>(READ_FIELD(this, kMapOffset)));
 }