Use smi zero instead of undefine_value to zap dead weak cells.

src/heap/objects-visiting-inl.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_OBJECTS_VISITING_INL_H_
 #define V8_OBJECTS_VISITING_INL_H_
 
 
 namespace v8 {
 namespace internal {
@@ skipping 341 matching lines @@
 }
 
 
 template <typename StaticVisitor>
 void StaticMarkingVisitor<StaticVisitor>::VisitWeakCell(Map* map,
                                                         HeapObject* object) {
   Heap* heap = map->GetHeap();
   WeakCell* weak_cell = reinterpret_cast<WeakCell*>(object);
   Object* undefined = heap->undefined_value();
   // Enqueue weak cell in linked list of encountered weak collections.
-  // We can ignore weak cells with cleared values because they will always point
-  // to the undefined_value.
-  if (weak_cell->next() == undefined && weak_cell->value() != undefined) {
+  // We can ignore weak cells with cleared values because they will always
+  // contain smi zero.
+  if (weak_cell->next() == undefined && !weak_cell->cleared()) {
     weak_cell->set_next(heap->encountered_weak_cells());
     heap->set_encountered_weak_cells(weak_cell);
   }
 }
 
 
 template <typename StaticVisitor>
 void StaticMarkingVisitor<StaticVisitor>::VisitAllocationSite(
     Map* map, HeapObject* object) {
   Heap* heap = map->GetHeap();
@@ skipping 569 matching lines @@
 
   RelocIterator it(this, mode_mask);
   for (; !it.done(); it.next()) {
     it.rinfo()->template Visit<StaticVisitor>(heap);
   }
 }
 }
 }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_VISITING_INL_H_