Exxon Mobil indicated Friday who would succeed Rex Tillerson as head of the country's largest oil company, tapping refining chief Darren W. Woods to be president. The move comes as Tillerson, 63, nears the company's mandatory retirement age of 65. Over the last year speculation has grown up around whether the Irving-based Exxon board would go with Woods, 50, or Jack P. Williams, the 52-year-old former executive vice president of the company's oil and gas production operation.