Yahoo said Chief Executive Marissa Mayer faced “specific security threats” in 2015 — and the company accordingly spent more than a half-million dollars on her personal security. Last year’s tab came to $544,061 — more than 20 times the $26,891 Yahoo had spent a year earlier on protecting Mayer and her immediate family, according to a Monday filing. “During 2015 Ms. Mayer faced specific security threats that we believed were credible,” Yahoo said in the filing. The company didn’t elaborate on the threats. By comparison, Facebook said last month it spent $6.5 million on bodyguards and other security for its...