The Grocery Manufacturers Association (GMA) — which represents food-industry giants including Coca-Cola, MARS, Kellogg, P&G, McDonald’s, Mondelez International, Starbucks, Hershey, General Mills and roughly 300 others — announced this week that it will petition to the chief U.S. food safety regulator and Congress to enact a single federal standard for the labeling of genetically modified (GMO) foods.