Factor out handling of mixed objects preprocessing after migration

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index e50c6ba77e1dfe71eb0f9ab2c1f02f95baf99655..528124d35b7007380a5117728fe18add57b002fc 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1516,6 +1516,8 @@ class Heap {
   inline void OnMoveEvent(HeapObject* target, HeapObject* source,
                           int size_in_bytes);
 
+  void MigrateMixedObjectInNewSpaceEpilog(HeapObject* dst);
+
   bool deserialization_complete() const { return deserialization_complete_; }
 
   void RegisterNewArrayBuffer(void* data, size_t length);