A 15-year-old boy from Chelsea has been identified as the victim of a fatal stabbing Sunday in East Boston.Irvin Depaz of Chelsea was stabbed on Trenton Street about 5:11 p.m., police said.He was taken to Massachusetts General Hospital, where he was later pronounced dead.Depaz was the second 15-year-old to be stabbed to death in East Boston in the past two weeks.On Sept.