Enable Null Syncable ID which is different than Root ID.

sync/engine/syncer_util.cc

Index: sync/engine/syncer_util.cc
diff --git a/sync/engine/syncer_util.cc b/sync/engine/syncer_util.cc
index 3cb9bcaf7ec4670de7a4aedadf97765999a09bd0..7f63b37ddbdeeeb0a654b54a12ef6a597b67e34a 100644
--- a/sync/engine/syncer_util.cc
+++ b/sync/engine/syncer_util.cc
@@ -121,7 +121,7 @@ syncable::Id FindLocalIdToUpdate(
             // we don't server delete the item, because we don't allow it to
             // exist locally at all.  So the item will remain orphaned on
             // the server, and we won't pay attention to it.
-            return syncable::GetNullId();
+            return syncable::Id();
           }
         }
         // Target this change to the existing local entry; later,