Treat links that organize weak objects weakly.

src/heap/objects-visiting-inl.h

Index: src/heap/objects-visiting-inl.h
diff --git a/src/heap/objects-visiting-inl.h b/src/heap/objects-visiting-inl.h
index ff26c76cd0c4d5b549ec228e7dbca1393ee994ee..3ef51866b6341715e424292e44a6c68a06356cc8 100644
--- a/src/heap/objects-visiting-inl.h
+++ b/src/heap/objects-visiting-inl.h
@@ -332,7 +332,8 @@ void StaticMarkingVisitor<StaticVisitor>::VisitWeakCell(Map* map,
   // 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());
+    weak_cell->set_next(heap->encountered_weak_cells(),
+                        UPDATE_WEAK_WRITE_BARRIER);
     heap->set_encountered_weak_cells(weak_cell);
   }
 }