The asset should be a gallery asset. So that would normally just be gallery.MEDIA_TYPE.MEDIA_ID. You should be able to just make a copy of the CB Gallery comment stream included with CB Activity then make whatever changes you need for that specific media then I assume you're using CB Auto Actions to insert that comment stream specifically for that media and in that auto action you'd add your conditions to check for profile owner and viewer user id against the asset of the media.