By Frankie Taddeo The Chargers have been strong on the road but face a tough challenge against the high-powered 49ers offense. Justin Herbert and the Chargers will put their perfect 4-0 against-the-spread (ATS) road record on the line in Week 10 when they head to San Francisco to take on Christian McCaffrey and the 49ers. Los Angeles is expected to be without both Keenan Allen (hamstring) and Mike WIlliams (ankle) once again. The absence of either one or both of his starting wideouts has led to Herbert failing to throw multiple touchdown passes in four of his last six games. Despite missing tw…