UPDATE: The previous information has been unconfirmed by Robert Bowling:
It is not confirmed that Riot Shield is a perk or that it takes up Perk 1 slot.
The debate continues.
Bowling has already confirmed the information two times (accidently), so this comes as a surprise. Nonetheless the Riot Shield will be an interesting asset in multiplayer, as quoted by Bowling:
Riot shield guys are defenseless (except they can melee with it) because the Riot Shield requires both hands to hold, so you can't shoot when you have it out. Best used in conjunction with another player. Even then, plenty of ways to counter someone with a shield.
It's really good in objective gametypes. When one players needs to plant / defuse / defend / etc. / etc. and the other player can protect him. Giving him some mobile cover while he works. It has it's advantages in TDM as well, but Objective gametypes is where I've really seen it shine. We can expect more information when the next multiplayer videos are released.