Combine first 10 ingredients in a small bowl; stir well. Arrange onion slices in a single layer in a shallow dish. Add chicken breast halves; pour herb mixture over onion and chicken. Cover and marinate in refrigerator 1 hour.
Pull husks back from corn (do not remove). Remove silks. Pull husks back over corn, and tie top with heavy string; set aside.
Prepare grill. Remove onion slices and chicken from dish, reserving marinade. Place chicken breast halves, bone sides up, on grill rack coated with cooking spray; grill 50 minutes or until chicken is done, turning and basting with reserved marinade every 10 minutes. Moisten corn with water; grill 10 minutes or until tender, turning corn after 5 minutes. Grill onion slices 6 minutes or until tender, turning and basting with reserved marinade after 3 minutes.