Warren Buffett's Berkshire Hathaway bought nearly $1 billion of shares in Activision Blizzard before Microsoft agreed to acquire it for $68.7 billion, according to a regulatory filing. Berkshire said it owned 14.7 million shares worth about $975 million of Activision as of December 31. Microsoft's plan to buy Activision Blizzard, which is its largest-ever acquisition, was announced in January.