Japan’s handset-toting masses, it seems, have a lot to teach the Net giant. The country has become a vast lab for Google as it tries to refine mobile search technology. That’s because Japan’s 100 million cell-phone users represent the most diverse—and discriminating—pool of mobile subscribers on the planet. While Google also does plenty of testing elsewhere, the Japanese are often more critical because they are as likely to tap into the Net with a high-tech phone as a PC and can do so at speeds rivaling fixed-line broadband. And because Japanese carriers have offered such services for years, plenty of Web sites are formatted for cell phones.