Factor out handling of mixed objects preprocessing after migration

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 322965decdd4e6d57a403f152c85ab8a07c26c16..6373a54f24a13a011133219327e44b593dc16ca7 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -648,6 +648,8 @@ class MarkCompactCollector {
       Object** anchor_slot, Object** slot, Object* object,
       SlotsBuffer::AdditionMode mode = SlotsBuffer::FAIL_ON_OVERFLOW));
 
+  void MigrateMixedObjectEpilog(HeapObject* dst);
+
   void MigrateObject(HeapObject* dst, HeapObject* src, int size,
                      AllocationSpace to_old_space);