Pittsburgh's H.J. Heinz Co said Warren Buffett's Berkshire Hathaway Inc. has become its majority shareholder by exercising a warrant ahead of the ketchup maker's planned merger with Northfield, Ill.-based Kraft Foods Group Inc. In a regulatory filing, Heinz said Berkshire exercised a warrant to acquire about 46.2 million shares for nearly $462,000 this week. Heinz said the shares represent about 5.4 percent of its outstanding common stock, and that their issuance gives Berkshire a 52.5 percent overall stake.