OAK BROOK, Ill., May 24 - McDonald's Corp investors soundly rejected a shareholder proposal that would have required the world's biggest fast-food chain to assess its impact on childhood obesity. The subject was a major topic of discussion at Thursday's annual shareholder meeting, which also served as a send-off for retiring Chief Executive Jim Skinner - whose nearly eight years at the helm will be remembered as a time when the price of McDonald's stock tripled. ...