[heap] Fix verification of unsafe object layout changes.

src/heap/concurrent-marking.cc

Index: src/heap/concurrent-marking.cc
diff --git a/src/heap/concurrent-marking.cc b/src/heap/concurrent-marking.cc
index 32afac63f3eae03b5e351ef33d9009e79372b0fb..318de2e0a4ec1b9cd182b2ab08b194ac9babbd25 100644
--- a/src/heap/concurrent-marking.cc
+++ b/src/heap/concurrent-marking.cc
@@ -193,6 +193,8 @@ class ConcurrentMarkingVisitor final
   };
 
   const SlotSnapshot& MakeSlotSnapshot(Map* map, HeapObject* object, int size) {
+    // TODO(ulan): Iterate only the existing fields and skip slack at the end
+    // of the object.
     SlotSnapshottingVisitor visitor(&slot_snapshot_);
     visitor.VisitPointer(object,
                          reinterpret_cast<Object**>(object->map_slot()));