CARSON, Calif. — Although Philip Rivers has only faced the Seattle Seahawks three times in games that count, he knows nearly everything about their defense because of Gus Bradley. Rivers used that know-how to get the Los Angeles Chargers off to a strong start in their first victory of the preseason. Melvin Gordon rushed for a short touchdown to cap a strong 70-yard opening drive by the Bolts’ first-teamers, and Rivers went 6 of 7 for 62 yards in Los Angeles’ 24-14 preseason victory over the Seahawks on Saturday night. Rivers played only two series in his first action of the preseason, but he had six consecutive completions.