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

src/objects.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <sstream>
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
@@ skipping 9713 matching lines @@
     return isolate->factory()->undefined_value();
   }
   return result;
 }
 
 
 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()));
     }
     // If we found an empty WeakCell, that means the script wrapper was
     // GCed.  We are not notified directly of that, so we decrement here
     // so that we at least don't count double for any given script.
     isolate->counters()->script_wrappers()->Decrement();
   }
   // Construct a new script wrapper.
   isolate->counters()->script_wrappers()->Increment();
@@ skipping 6664 matching lines @@
   Handle<DependentCode> codes =
       DependentCode::Insert(handle(cell->dependent_code(), info->isolate()),
                             DependentCode::kPropertyCellChangedGroup,
                             info->object_wrapper());
   if (*codes != cell->dependent_code()) cell->set_dependent_code(*codes);
   info->dependencies(DependentCode::kPropertyCellChangedGroup)->Add(
       cell, info->zone());
 }
 
 } }  // namespace v8::internal