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

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index a5b46a454a15b948d9cc649a125243d6cf1604aa..26adbc12886480112a3173afe4864e4ec705af27 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1930,13 +1930,14 @@ void PropertyCell::set_type_raw(Object* val, WriteBarrierMode ignored) {
 }
 
 
-HeapObject* WeakCell::value() const {
+Object* WeakCell::value() const {
   return HeapObject::cast(READ_FIELD(this, kValueOffset));
 }
 
 
-void WeakCell::clear(HeapObject* undefined) {
-  WRITE_FIELD(this, kValueOffset, undefined);
+void WeakCell::clear() {
+  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT);
+  WRITE_FIELD(this, kValueOffset, Smi::FromInt(0));
 }
 
 
@@ -1946,6 +1947,9 @@ void WeakCell::initialize(HeapObject* val) {
 }
 
 
+bool WeakCell::cleared() const { return value() == Smi::FromInt(0); }
+
+
 Object* WeakCell::next() const { return READ_FIELD(this, kNextOffset); }