SEATTLE — After hiring the youngest head coach in the NFL this week in Mike Macdonald, it was expected the Seahawks would try to add some veteran savvy to their assistant coaching staff to help him out. And that’s exactly what the Seahawks did Friday in coming to an agreement to hire Leslie Frazier as assistant head coach — a coach who at 64 is not only almost twice as old as Macdonald’s 36, but has also been considered something of a mentor to him . A league source confirmed the expected hire to The Seattle Times. Frazier, who may best be known as the head coach of the Vikings from 2010-13, w…