Put salmon in a bowl; remove skin and bones. In a bowl, whisk egg whites and mayonnaise. Stir all ingredients together (except buns). Form into 8 patties, cover and chill for 1 hour. Prepare a charcoal fire or set a gas grill to medium-high. Thoroughly oil grill grate; mist burgers with cooking spray. Grill burgers until firm, 3 minutes per side.