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

test/cctest/test-heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4292 matching lines @@
   }
   CHECK(weak_cell1->value()->IsFixedArray());
   CHECK_EQ(*survivor, weak_cell2->value());
   heap->CollectGarbage(NEW_SPACE);
   CHECK(weak_cell1->value()->IsFixedArray());
   CHECK_EQ(*survivor, weak_cell2->value());
   heap->CollectGarbage(NEW_SPACE);
   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());
 }
 
 
 TEST(WeakCellsWithIncrementalMarking) {
   CcTest::InitializeVM();
   Isolate* isolate = CcTest::i_isolate();
   v8::internal::Heap* heap = CcTest::heap();
   v8::internal::Factory* factory = isolate->factory();
 
   const int N = 16;
@@ skipping 10 matching lines @@
     IncrementalMarking* marking = heap->incremental_marking();
     if (marking->IsStopped()) marking->Start();
     marking->Step(128, IncrementalMarking::NO_GC_VIA_STACK_GUARD);
     heap->CollectGarbage(NEW_SPACE);
     CHECK(weak_cell->value()->IsFixedArray());
     weak_cells[i] = inner_scope.CloseAndEscape(weak_cell);
   }
   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());
   }
 }
 
 
 #ifdef DEBUG
 TEST(AddInstructionChangesNewSpacePromotion) {
   i::FLAG_allow_natives_syntax = true;
   i::FLAG_expose_gc = true;
   i::FLAG_stress_compaction = true;
   i::FLAG_gc_interval = 1000;
@@ skipping 255 matching lines @@
 #ifdef DEBUG
 TEST(PathTracer) {
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
 
   v8::Local<v8::Value> result = CompileRun("'abc'");
   Handle<Object> o = v8::Utils::OpenHandle(*result);
   CcTest::i_isolate()->heap()->TracePathToObject(*o);
 }
 #endif  // DEBUG