Warren Buffett’s Berkshire Hathaway Inc. acquired nearly $1 billion of shares in Activision Blizzard Inc. before Microsoft Corp. agreed to buy the video game maker for $68.7 billion, according to a Monday regulatory filing. Berkshire said that as of Dec. 31, it owned 14.7 million shares worth about $975 million of the “Call of Duty” maker. Microsoft announced its plan to buy Activision Blizzard on Jan. 18, in its largest-ever acquisition. Activision shares are up 23 percent this year to $81.50, though they remain well below the proposed $95 per share takeover price, reflecting potential antitrust concerns. Another prominent investor, Daniel Loeb’s hedge fund Third Point, bought 2 million Activision shares in the fourth quarter. Berkshire disclosed its Activision stake in a filing detailing its U.S.-listed stock investments as of Dec. 31. Investors monitor Berkshire’s investments closely to see where Buffett and his investment managers Todd Combs and Ted Weschler …