Serializer: canonicalize cleared weak cells.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 9dd4b1466272b534b98c2f10d97a8855c749dd77..5ac70f872a3c9297bac5c50d539f1a9ed61353e2 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -185,7 +185,8 @@ namespace internal {
   V(FixedArray, keyed_load_dummy_vector, KeyedLoadDummyVector)                 \
   V(FixedArray, detached_contexts, DetachedContexts)                           \
   V(ArrayList, retained_maps, RetainedMaps)                                    \
-  V(WeakHashTable, weak_object_to_code_table, WeakObjectToCodeTable)
+  V(WeakHashTable, weak_object_to_code_table, WeakObjectToCodeTable)           \
+  V(WeakCell, cleared_weak_cell, ClearedWeakCell)
 
 // Entries in this list are limited to Smis and are not visited during GC.
 #define SMI_ROOT_LIST(V)                                                   \