Chrome OS is at last gaining some momentum — among US businesses and schools, anyway. The NPD Group reports that Chromebooks represented 9.6 percent of all computing devices sold through American commercial channels in 2013, or enough to surpass the market share of Android tablets, Windows tablets and MacBooks. Googles rise put the squeeze on traditional computers, which dropped from 77.8 percent of the commercial space in 2012 to 63.7 percent this year.