Schiff Hardin LLP, one of the oldest and largest law firms based in Chicago, and Arent Fox LLP announced Wednesday that they would merge, creating the new firm ArentFox Schiff as of March 1.ArentFox Schiff will have 600 lawyers and policy professionals based in seven U.S. markets: Chicago, Washington, D.C., New York, Los Angeles, Boston, San Francisco and Ann Arbor, Michigan.The firm will be led by chair Anthony V. Lupo, now chair of Arent Fox, and firmwide co-managing partners Cristina A. Carvalho, now with Arent Fox in …