Arlington police arrested 21 protesters yesterday after they chained themselves to the front door and one another at Bush-Cheney campaign headquarters during a demonstration against administration policies. About 120 protesters took part in the demonstration outside the building on Wilson Boulevard that houses the campaign headquarters, police said. The protest began about 2 p.m. and lasted 90 minutes, they said. The protest was organized by ACT UP and Housing Works, a nonprofit agency that provides social services, housing and health care to people with AIDS in New York, said Robert Cordero, director of federal advocacy for Housing Works. "We...