Joseph Robinette "Beau" Biden III, the son of Vice President Joe Biden and former state attorney general of Delaware, died Saturday after battling brain cancer for several years.Biden, 46, the oldest son of the vice president and the rising star of a family dynasty, had been admitted recently to Walter Reed National Military Medical Center in Bethesda, Maryland, as he fought the cancer, a battle that his father largely kept private in the last weeks as his son clung to his life.