Reuters: A group of investors including the two largest U.S. public pension funds asked 27 top oil and gas companies on Thursday to disclose what they have done to make their offshore drilling safer in the wake of the BP (BP.L)(BP.N) oil spill. Members of the Ceres coalition of socially-concerned investors, who manage some $2.5 trillion in assets, called on companies including Exxon Mobil Corp (XOM.N) and Chevron Corp (CVX.N) to say what steps they are taking to make sure their offshore …