Robert L. Barry, an American diplomat who was the chief U.S. negotiator in reaching a key military agreement with the Soviet Union near the end of the Cold War, died on March 11 at his home in Newton, Mass. He was 89.His wife, Margaret Barry, said the cause was multi-infarct dementia.Mr. Barry led a U.S. negotiating team at a security conference in Stockholm in the summer of 1986, when he and his Soviet counterpart, Oleg Grinevsky, reached an accord on troop inspections that American officials regarded as important in reducing East-West tensions.The agreement established that NATO and…