After battling for months with The Boeing Co.'s leadership, the company's largest union approved an eight-year contract that trades hard-fought pension benefits for the right to build the 777X airliner — a bitterly fought concession that underscores unions' uphill battle at the bargaining table. It's part of a larger trend for labor unions, which face declines in membership strength, reduced bargaining power because of right-to-work states and hostile public opinion.