Ensure that all non-stable maps created by Map::AddMissingTransitions() are marked as such.

test/cctest/test-unboxed-doubles.cc

Index: test/cctest/test-unboxed-doubles.cc
diff --git a/test/cctest/test-unboxed-doubles.cc b/test/cctest/test-unboxed-doubles.cc
index 7125f632d971cc3319c9146ac1a7a62a7a28ada0..3906d848de4f24ecfd24da18a39803f76939acba 100644
--- a/test/cctest/test-unboxed-doubles.cc
+++ b/test/cctest/test-unboxed-doubles.cc
@@ -759,12 +759,14 @@ static Handle<LayoutDescriptor> TestLayoutDescriptorAppendIfFastOrUseFull(
   int descriptors_length = descriptors->number_of_descriptors();
   std::vector<Handle<Map>> maps(descriptors_length);
   {
+    CHECK(last_map->is_stable());
     Map* map = *last_map;
     for (int i = 0; i < descriptors_length; i++) {
       maps[descriptors_length - 1 - i] = handle(map, isolate);
       Object* maybe_map = map->GetBackPointer();
       CHECK(maybe_map->IsMap());
       map = Map::cast(maybe_map);
+      CHECK(!map->is_stable());
     }
     CHECK_EQ(1, maps[0]->NumberOfOwnDescriptors());
   }