Just add slots that point to to-space objects back to the store buffer.

src/heap/store-buffer.h

Index: src/heap/store-buffer.h
diff --git a/src/heap/store-buffer.h b/src/heap/store-buffer.h
index 5efd6922bcd6bc30f3a21c298761ae52beeb8513..30ef18f54f08f9c924f206dfde212e6c55c77e1f 100644
--- a/src/heap/store-buffer.h
+++ b/src/heap/store-buffer.h
@@ -148,6 +148,9 @@ class StoreBuffer {
   // Set the map field of the object to NULL if contains a map.
   inline void ClearDeadObject(HeapObject* object);
 
+  void ProcessOldToNewSlot(Address slot_address,
+                           ObjectSlotCallback slot_callback, bool clear_maps);
+
   void IteratePointersToNewSpace(ObjectSlotCallback callback, bool clear_maps);
 
   void FindPointersToNewSpaceInRegion(Address start, Address end,