Fix benchRoundtrip benchmark
Fix benchRoundtrip benchmark

Benchmarking the cost of roundtripping the demo database got broken by deleting the demo databse.  Oops...

Benchmark the cost of roundtripping core instead.

google/CodeCity,google/CodeCity,NeilFraser/CodeCity,google/CodeCity,NeilFraser/CodeCity,NeilFraser/CodeCity,NeilFraser/CodeCity,google/CodeCity