LOS ANGELES — Biotech billionaire Patrick Soon-Shiong has agreed to purchase the Los Angeles Times from its parent company Tronc, restoring local ownership and perhaps ending a turbulent period for the storied 136-year-old institution. Chicago-based Tronc on Wednesday will announce the sale of The Times and the San Diego Union-Tribune to Soon-Shiong’s investment firm Nant Capital for nearly $500 million in cash, according to a source familiar with the deal.