Sir Paul McCartney enjoyed a very special treat to mark his 79th birthday on Friday courtesy of his daughters Mary and Stella McCartney.The duo exercised their vocal cords to sing 'Happy Birthday' to their famous dad in a sweet video shared on Instagram.MORE: Paul McCartney and wife Nancy Shevell enjoy romantic trip to the beachIn the clip, photographer Mary, 51, and fashion designer Stella, 49, belted out the classic song to the camera before panning over to capture Paul and enjoy a sweet