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

test/cctest/test-heap.cc

Index: test/cctest/test-heap.cc
diff --git a/test/cctest/test-heap.cc b/test/cctest/test-heap.cc
index 2e285916079f61528887345627c52b2d656e0168..14490500f135af3a01bfc42af062665ea26cecc3 100644
--- a/test/cctest/test-heap.cc
+++ b/test/cctest/test-heap.cc
@@ -4312,8 +4312,8 @@ TEST(WeakCell) {
   CHECK(weak_cell1->value()->IsFixedArray());
   CHECK_EQ(*survivor, weak_cell2->value());
   heap->CollectAllAvailableGarbage();
+  CHECK(weak_cell1->cleared());
   CHECK_EQ(*survivor, weak_cell2->value());
-  CHECK(weak_cell2->value()->IsFixedArray());
 }
 
 
@@ -4344,7 +4344,7 @@ TEST(WeakCellsWithIncrementalMarking) {
   heap->CollectAllGarbage(Heap::kNoGCFlags);
   CHECK_EQ(*survivor, weak_cells[0]->value());
   for (int i = 1; i < N; i++) {
-    CHECK(weak_cells[i]->value()->IsUndefined());
+    CHECK(weak_cells[i]->cleared());
   }
 }