Add map-based read barrier to WeakCell

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 2975d09f63de15cb16e33a0d1c19ba030b5fc976..59434548afb50b32604767dd30289e990fb23e46 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -52,7 +52,8 @@ namespace internal {
   V(Map, fixed_cow_array_map, FixedCOWArrayMap)                                \
   V(Map, fixed_double_array_map, FixedDoubleArrayMap)                          \
   V(Map, constant_pool_array_map, ConstantPoolArrayMap)                        \
-  V(Map, weak_cell_map, WeakCellMap)                                           \
+  V(Map, used_weak_cell_map, UsedWeakCellMap)                                  \
+  V(Map, unused_weak_cell_map, UnusedWeakCellMap)                              \
   V(Oddball, no_interceptor_result_sentinel, NoInterceptorResultSentinel)      \
   V(Map, hash_table_map, HashTableMap)                                         \
   V(Map, ordered_hash_table_map, OrderedHashTableMap)                          \
@@ -342,7 +343,8 @@ namespace internal {
   V(FixedCOWArrayMap)                   \
   V(FixedDoubleArrayMap)                \
   V(ConstantPoolArrayMap)               \
-  V(WeakCellMap)                        \
+  V(UsedWeakCellMap)                    \
+  V(UnusedWeakCellMap)                  \
   V(NoInterceptorResultSentinel)        \
   V(HashTableMap)                       \
   V(OrderedHashTableMap)                \