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

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 055d0d7f1f5e74047214381593df045510a814e2..7ab6596453db35e1c4447696a924382cefe6c189 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -191,6 +191,7 @@ namespace internal {
   V(Object, noscript_shared_function_infos, NoScriptSharedFunctionInfos)       \
   V(FixedArray, interpreter_table, InterpreterTable)                           \
   V(Map, bytecode_array_map, BytecodeArrayMap)                                 \
+  V(WeakCell, empty_weak_cell, EmptyWeakCell)                                  \
   V(BytecodeArray, empty_bytecode_array, EmptyBytecodeArray)
 
 
@@ -448,6 +449,7 @@ namespace internal {
   V(JSMessageObjectMap)                 \
   V(ForeignMap)                         \
   V(NeanderMap)                         \
+  V(EmptyWeakCell)                      \
   V(empty_string)                       \
   PRIVATE_SYMBOL_LIST(V)