UNDER normal circumstances, a nasty public power struggle between a company’s controlling shareholder and its chief executive might put a dampener on the share price. But the circumstances at Viacom, a media conglomerate, are anything but normal. Sumner Redstone (pictured), an ailing 92-year-old mogul, recently kept control of his $42 billion media empire after a humiliating legal battle to prise it from his hands.