[serializer] prepare attached references for general use.

src/snapshot/startup-serializer.cc

Index: src/snapshot/startup-serializer.cc
diff --git a/src/snapshot/startup-serializer.cc b/src/snapshot/startup-serializer.cc
index fab01f51f801d4db83f3f48d5d5405cfc86f6f3d..c3f9b3eb61d50e7e8bc6acf9846c1aa46ffe4338 100644
--- a/src/snapshot/startup-serializer.cc
+++ b/src/snapshot/startup-serializer.cc
@@ -73,8 +73,8 @@ void StartupSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
     // Make sure that the immortal immovable root has been included in the first
     // chunk of its reserved space , so that it is deserialized onto the first
     // page of its space and stays immortal immovable.
-    BackReference ref = back_reference_map_.Lookup(obj);
-    CHECK(ref.is_valid() && ref.chunk_index() == 0);
+    SerializerReference ref = reference_map_.Lookup(obj);
+    CHECK(ref.is_back_reference() && ref.chunk_index() == 0);
   }
 }