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

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 5e9e154ff23ceabe7f2ba8f5b7972745ae2db2a7..e6f5e886d7e29d2ac650874002c20dc09ec6f2cd 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -9731,7 +9731,7 @@ Handle<JSObject> Script::GetWrapper(Handle<Script> script) {
   Isolate* isolate = script->GetIsolate();
   if (!script->wrapper()->IsUndefined()) {
     Handle<WeakCell> cell(WeakCell::cast(script->wrapper()));
-    if (!cell->value()->IsUndefined()) {
+    if (!cell->cleared()) {
       // Return a handle for the existing script wrapper from the cache.
       return handle(JSObject::cast(cell->value()));
     }