Use WeakCells in the optimized code map rather than traversing in pause.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index 75467cb1255fa4c89602fbdbe38a2f50fae8b160..8c102ed869a50230ddcca3ac7d68ae6e3ff72a2f 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -227,6 +227,7 @@ enum BindingFlags {
   V(SCRIPT_CONTEXT_TABLE_INDEX, ScriptContextTable, script_context_table)      \
   V(SCRIPT_FUNCTION_INDEX, JSFunction, script_function)                        \
   V(SECURITY_TOKEN_INDEX, Object, security_token)                              \
+  V(SELF_WEAK_CELL_INDEX, WeakCell, self_weak_cell)                            \
   V(SET_ITERATOR_MAP_INDEX, Map, set_iterator_map)                             \
   V(SHARED_ARRAY_BUFFER_FUN_INDEX, JSFunction, shared_array_buffer_fun)        \
   V(SLOPPY_ARGUMENTS_MAP_INDEX, Map, sloppy_arguments_map)                     \