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

src/heap/store-buffer.h

 // Copyright 2011 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.
 
 #ifndef V8_STORE_BUFFER_H_
 #define V8_STORE_BUFFER_H_
 
 #include "src/allocation.h"
 #include "src/base/logging.h"
 #include "src/base/platform/platform.h"
@@ skipping 130 matching lines @@
 
   void ClearFilteringHashSets();
 
   bool SpaceAvailable(intptr_t space_needed);
   void Uniq();
   void ExemptPopularPages(int prime_sample_step, int threshold);
 
   // 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,
                                       ObjectSlotCallback slot_callback,
                                       bool clear_maps);
 
   // For each region of pointers on a page in use from an old space call
   // visit_pointer_region callback.
   // If either visit_pointer_region or callback can cause an allocation
   // in old space and changes in allocation watermark then
@@ skipping 51 matching lines @@
   }
 
  private:
   StoreBuffer* store_buffer_;
   bool stored_state_;
 };
 }
 }  // namespace v8::internal
 
 #endif  // V8_STORE_BUFFER_H_