I don't know who'd win, but the number of people saying Chris Robshaw is quite surprising. I definitely think it'd be a lot closer than some people are making it out to be. Pacquiao is not only a professional boxer, he's one of the best. I agree that if Robshaw got him to the ground, Pacquiao's probably lost, but it'd be a big task in itself just to get him there, even for a rugby player. A boxer's footwork and balance is phenomenal - he's not going to stand there and get charged down.
The other point made was that Pacquiao doesn't have the reach to even be able to punch Robshaw and if he got close enough, he'd just get grabbed. Again, i doubt it's that clear cut. Pacquiao is quick - REALLY quick. He could probably put away a combo before Robshaw could react. I don't know know where the fight would go from there, either Pacquiao's punches are enough to daze/hurt Robshaw (which is probably more likely tbh) or Robshaw can take a few hits and manage to grab him.