Coco Gauff is still a teenager, after all, and so it should surprise no one that she was on her phone in the locker room, scrolling through social media, right up until 10 minutes before heading out on court for the U.S. Open final.   What the 19-year-old from Delray Beach was reading, she would say later, were various comments, negative ones, “saying I wasn’t going to win today; that just put the fire in me.”  Setting aside a so-so start Saturday, Gauff surged to a 2-6, 6-3, 6-2 victory over the soon-to-be-No. 1 Aryna Sabalenka in the final at Arthur Ashe Stadium, delighting a raucous crowd that backed her from start to finish.