I asked a professional processor about the growth today (while he had one of my birds with such a growth in front of him). He called it a callus and then qualified by saying maybe that isn't the right word. He cut it off suggesting that I wouldn't want the liquid inside it to run out during roasting.
I'm still a bit curious because this growth seemed to be a living part of the chicken rather than what I think of as a callus (being dead since it can be filed or shaved).
The person I spoke to said it wasn't caused by constant time on the ground, but rather was more common in birds that roosted in coops with hard floors, when they jump off the roost they may injure their breast.