Gov. Gavin Newsom, diving into one of California’s most contentious water issues, said Tuesday he wants to downsize the Delta tunnels project. The Democratic governor also set out to overhaul state water policy by naming a new chair of the state’s water board. Newsom said he wants the twin-tunnel project — designed to re-engineer the troubled estuary that serves as the hub of California’s elaborate water-delivery system — reduced to a single tunnel.