Optimize clearing of map transitions.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index 5aa464f34eee9e7916e0b637687ccd483af6ff0f..3dace163183954c02abeffee65bb2e7ec269c72c 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -652,6 +652,7 @@ void IncrementalMarking::ProcessWeakCells() {
   DCHECK(!finalize_marking_completed_);
   DCHECK(IsMarking());
 
+  Object* the_hole_value = heap()->the_hole_value();
   Object* weak_cell_obj = heap()->encountered_weak_cells();
   Object* weak_cell_head = Smi::FromInt(0);
   WeakCell* prev_weak_cell_obj = NULL;
@@ -671,7 +672,7 @@ void IncrementalMarking::ProcessWeakCells() {
         prev_weak_cell_obj->set_next(weak_cell->next());
       }
       weak_cell_obj = weak_cell->next();
-      weak_cell->clear_next(heap());
+      weak_cell->clear_next(the_hole_value);
     } else {
       if (weak_cell_head == Smi::FromInt(0)) {
         weak_cell_head = weak_cell;