WASHINGTON — Key senators announced a framework agreement on new gun legislation Sunday, marking a breakthrough on a collection of measures to combat gun violence, including “red flag” laws and enhanced background checks on buyers. The chief negotiators of the deal are Sen. Chris Murphy, D-Conn., an outspoken proponent of gun safety laws, and Sen. John Cornyn, R-Texas, a firm Second Amendment advocate who has promised the new measures won’t impact the gun rights of law-abiding Americans. The final bill hasn’t been written yet, sources familiar with the negotiations said. “Today, we are announcing a commonsense, bipartisan proposal to protect...