In “On Chesil Beach,” British writer Ian McEwan (“Atonement”) adapts his critically acclaimed 2007 novel of a crumbling marriage into a film that stars two-time Oscar nominee Saoirse Ronan (“Lady Bird”) and Billy Howle (“Dunkirk”) as the ill-fated lovers. When the movie opens, it’s 1962 London, and a pair of attractive young newlyweds -- Florence (Ronan, luminous, per usual) and Edward [...]