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

src/objects-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.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 2018 matching lines @@
 
 void PropertyCell::set_property_details(PropertyDetails details) {
   set_property_details_raw(details.AsSmi());
 }
 
 
 Object* WeakCell::value() const { return READ_FIELD(this, kValueOffset); }
 
 
 void WeakCell::clear() {
-  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT);
+  // Either the garbage collector is clearing the cell or we are simply
+  // initializing the root empty weak cell.
+  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT ||
+         this == GetHeap()->empty_weak_cell());
   WRITE_FIELD(this, kValueOffset, Smi::FromInt(0));
 }
 
 
 void WeakCell::initialize(HeapObject* val) {
   WRITE_FIELD(this, kValueOffset, val);
   Heap* heap = GetHeap();
   // We just have to execute the generational barrier here because we never
   // mark through a weak cell and collect evacuation candidates when we process
   // all weak cells.
@@ skipping 5827 matching lines @@
 #undef WRITE_INT64_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_OBJECTS_INL_H_