It's irrelevant. Shooters have been stopped by unarmed people talking to them. Shooters have also been stopped by shooting. That should not be the decision point on whether to allow guns on a site or not.
As shown by the op and by other stories - guns falling out of pockets in theaters and whatnot - the decision to allow guns or not has to include the chance of those guns being lost or taken let's say by a kid at school, etc.
Arming anyone to stop a mass shooting at one particular facility - since there may never be a shooting there and since they may do no good if there is one- you have to consider safety factors.
I assume at the Naval yard those who were armed did not leave their guns lying around in a bathroom.
At schools, offices, movie theaters, capitol buildings - the chances of accidents are so much greater than the chance of ever stopping a shooter, that allowing guns to be carried is stupid.