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

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 173 matching lines @@
   V(ArrayList, retained_maps, RetainedMaps)                                    \
   V(WeakHashTable, weak_object_to_code_table, WeakObjectToCodeTable)           \
   V(PropertyCell, array_protector, ArrayProtector)                             \
   V(PropertyCell, empty_property_cell, EmptyPropertyCell)                      \
   V(Object, weak_stack_trace_list, WeakStackTraceList)                         \
   V(Object, code_stub_context, CodeStubContext)                                \
   V(JSObject, code_stub_exports_object, CodeStubExportsObject)                 \
   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)
 
 
 // Entries in this list are limited to Smis and are not visited during GC.
 #define SMI_ROOT_LIST(V)                                                   \
   V(Smi, stack_limit, StackLimit)                                          \
   V(Smi, real_stack_limit, RealStackLimit)                                 \
   V(Smi, last_script_id, LastScriptId)                                     \
   V(Smi, arguments_adaptor_deopt_pc_offset, ArgumentsAdaptorDeoptPCOffset) \
   V(Smi, construct_stub_deopt_pc_offset, ConstructStubDeoptPCOffset)       \
@@ skipping 237 matching lines @@
   V(ScriptContextMap)                   \
   V(UndefinedMap)                       \
   V(TheHoleMap)                         \
   V(NullMap)                            \
   V(BooleanMap)                         \
   V(UninitializedMap)                   \
   V(ArgumentsMarkerMap)                 \
   V(JSMessageObjectMap)                 \
   V(ForeignMap)                         \
   V(NeanderMap)                         \
+  V(EmptyWeakCell)                      \
   V(empty_string)                       \
   PRIVATE_SYMBOL_LIST(V)
 
 // Forward declarations.
 class ArrayBufferTracker;
 class GCIdleTimeAction;
 class GCIdleTimeHandler;
 class GCIdleTimeHeapState;
 class GCTracer;
 class HeapObjectsFilter;
@@ skipping 2274 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_