I would think that after a while, your chickens would have found all the small stones that was really accessible/available to them in the enclosed run. I would still provide them with some easy grit by putting it in a dish or just taking a handful and throwing it in the run every two weeks or every week or so... depends on how many chickens you have in it, I would suppose.
If possible, maybe if you alternated free ranging them and keeping them in the run, then I think it would be okay. I free range my girls and I have grit available to them... I don't know where you live, but here, in the wintertime it seems to me they are quite thankful that I have grit available to them inside the coop. They will eat it if I put it out for them even though they free range.