( The Rev. Al Sharpton and family members of Eric Garner stressed their support of nonviolent protests following the deadly shootings of two New York City police officers. ) The ambush killing of two New York police officers Saturday has forced a burgeoning protest movement over police use of lethal force to address accusations that it bears some responsibility for violence carried out in the name of that cause. Officers Wenjian Liu and Rafael Ramos were fatally shot Saturday as they sat in their patrol car in a Brooklyn neighborhood. The suspect, 28-year-old Ismaaiyl Brinsley, is thought to have posted...