By Sarah McBride SAN FRANCISCO (Reuters) - Two of the nation's biggest online food delivery businesses said they are merging in a deal that they hope will drive more orders, in more cities, through their platforms. GrubHub and Seamless, which allow consumers to easily order online from various restaurants, are part of a group of fast-growing businesses that standardize local services under a national umbrella. Think restaurant reservations, where OpenTable dominates, or car services, where privately held startups such as Uber are making significant inroads. ...