CHONGQING, China-A surprise sales slump is hitting what had been a fast-growing part of the world's biggest car market. That could be bad news for mass-market global brands like Volkswagen The global auto industry has, in recent years, been betting on China's west, where they see potential for faster growth than in the more affluent-and car-saturated-cities along the coast and in the country's eastern and southern manufacturing belts.