I don't think Johnson had a glass jaw either. However he would not be able to handle the swarming pressure that Jack Dempsey would put on him. Dempsey's onslaught was like a vicious tornado. I think Johnson would be quickly hurt by Dempsey's body punches and grounded by his explosive left hook. Dempsey KO Round 1.
Some say Johnson had a glass jaw because Stanley Ketchel (a middleweight) floored him hard and almost knocked him out but it was a punch that caught Jack by surprise. They were following a script and then Stanley walloped Johnson for real.
Having said that I must also point out that though Jack didn't have a bad chin, it wasn't durable enough to withstand a rampaging Jack Dempsey. Dempsey KO 1.
If they fought 3 times Dempsey would win two out of three. Jack Johnson was a master at feinting and counterpunching. If Dempsey got careless for one second, that would be the end of the fight. Dempsey had a few poor habits like fighting with his left hand low and telegraphing his hooks; not a good idea against Johnson. But I imagine Dempsey's speed and big punching power would enable him to knock out Johnson before he could capitalize on anything.