The U.S. Treasury doesn’t get to invest in the booming stock market, but its stake in two guarantors of mortgage-backed securities is making a killing. Fannie Mae and Freddie Mac posted $9.9 billion in combined profits for the fourth quarter of 2016, and $20.2 billion for the full year. It was up 16.1 percent from 2015 and the fourth best year ever for the two government-sponsored enterprises. Their all-time high was...