Japanese automakers' U.S. sales slid mostly sideways last year, but their China business is booming. Two manufacturers, Mazda Motor Corp. and Mitsubishi Motors Corp., finally are selling more vehicles in the world’s biggest market than in the United States. Last year, Mazda’s passenger vehicle sales in China grew 8.3 percent to 309,407 vehicles, while its U.S. sales slid 2.8 percent. Mitsubishi’s sales in China also eclipsed those in the U.S., growing 56 percent to 129,