Don't unmark dictionary mode maps as unstable.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 1ea9441b7e5b3507f37075f87f862fdb53639d32..9ac2d6b7f48a60bb2382dd94d0e11deadea85878 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -3988,8 +3988,9 @@ bool Map::is_shared() {
 
 
 void Map::set_dictionary_map(bool value) {
-  if (value) mark_unstable();
-  set_bit_field3(DictionaryMap::update(bit_field3(), value));
+  uint32_t new_bit_field3 = DictionaryMap::update(bit_field3(), value);
+  new_bit_field3 = IsUnstable::update(new_bit_field3, value);
+  set_bit_field3(new_bit_field3);
 }