OSLO (Reuters) - Seadrill Chairman John Fredriksen, the oil rig firm's top shareholder, is stepping down with immediate effect and the company will write down asset values by more than $300 million (232.09 million pounds).The 75-year-old billionaire investor, who said his support for the company remained unchanged, would be replaced by Glen Ole Roedland, who has worked in shipping, oil, gas and other industries, Seadrill said.The company, which is listed in Oslo and New York, emerged from U.S...