Gamblers anxious to hit the slot machines and table games in person after a year of coronavirus restrictions are lifting the U.S. casino industry to its best year ever. But the great unknown is whether a new, more contagious variant of COVID-19 will slow an industry that is now winning money at a record pace. Data released Tuesday by the American Gaming Association, the casino industry’s national trade group, show that America’s casinos had their best second quarter in history, taking in $13.6 billion.