Shouldn't have much problems with that gpu playing watchdogs on lower settings, although just keep in mind that this is made by ubisoft and there has been a real trend lately of them focusing a lot more towards consoles and not caring about pc, meaning the game is not optimized and could be difficult to run. Assassins creed: Black Flag (also made by ubisoft) runs terrible on pc, I have R9 280x and I struggled to play it with 40 frames.
Just a heads up
EDIT: sorry I didnt realise it was at 1080p, your 850m even on low settings at that resolution you might manage just 30 fps.