Strawberries have a very mild taste. I've thought about this as well, but you need to find the juiciest, strongest tasting strawberries you can. Strawberry season is coming up soon so you might want to put it off until peak season when you can get the most flavorful berries. Buy them locally if you can, you'll get the better flavor then. If you buy them from the grocer they're usually picked green and don't have the flavor you want. Even then the chocolet and stout flavors might overpower the taste. If you want to do a strawberry beer, I'd suggest going for a lighter flavored beer. Or if you still want to do a stout, go light on the hops. They'll cover over the taste. Ususally the fruits that work best in stouts are the stronger flavored fruits, i.e. cherries, blueberries, ect.
I'm not telling you to "not do it" but check into the flavor you want to add. Also you might not even have to add chocolet to get the chocolet covered strawberry flavor. You might be able to just add the strawberries.
Good luck. Let us know how it's coming.