LOS ANGELES, Feb 27 (Reuters) - U.S. customs officers at a California border crossing seized more than 15 tons of marijuana hidden inside a tractor-trailer shipment designated as a cargo of mattresses, the biggest narcotics bust ever at that port of entry, officials said on Friday. Plastic-wrapped packages of marijuana, with a street value estimated at nearly $19 million, were found stacked floor to ceiling inside a trailer at the Otay Mesa cargo port in San Diego on Thursday, according to the U.S.