Workers at more than a dozen Chicago area Starbucks were on strike Christmas Eve, union organizers said, as baristas escalated a national walkout that began in three cities, including Chicago, on Friday. Across the country, baristas at more than 300 cafes were striking, according to their union, Starbucks Workers United. The union had previously warned that the strike, which began with walkouts in Chicago, Los Angeles and the coffee giant’s hometown of Seattle last week, could spread to hundreds of stores if the company did not meet the baristas’ demands at the bargaining table.