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

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index f53fb6c177c675e4e74c685f9633d011398220f6..a786f2df7168dbac9d7c337958cea183d13f4483 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2744,9 +2744,9 @@ void MarkCompactCollector::ProcessAndClearWeakCells() {
   Object* weak_cell_obj = heap()->encountered_weak_cells();
   while (weak_cell_obj != Smi::FromInt(0)) {
     WeakCell* weak_cell = reinterpret_cast<WeakCell*>(weak_cell_obj);
-    HeapObject* value = weak_cell->value();
+    Object* value = weak_cell->value();
     if (!MarkCompactCollector::IsMarked(value)) {

[Erik Corry Chromium.org, 2014/10/20 08:00:17]

IsMarked will assert that the value is a heap object, so you should handle the
non-heap-object case here or make value a HeapObject pointer and use
HeapObject::cast() if you don't think it can happen.

[ulan, 2014/10/20 12:30:18]

Done.

-      weak_cell->clear(undefined);
+      weak_cell->clear();
     } else {
       Object** slot = HeapObject::RawField(weak_cell, WeakCell::kValueOffset);
       heap()->mark_compact_collector()->RecordSlot(slot, slot, value);