They were a part of history, bringing the biggest news events — from Watergate, to the 9/11 terror attacks and raging local wildfires — to San Diegans’ doorsteps. And now, the presses at The San Diego Union-Tribune are themselves history, with the final print run in Mission Valley coming to a close at 1 a.m. Sunday. Starting with Monday’s paper, the Union-Tribune is being printed in Los Angeles, a byproduct of the U-T’s sale last month to the Tribune Publishing Co., parent company of the Los Angeles Times.