George Gallup and Lou Harris may have their scientific methods for conducting polls -- but Don McGaffin of Seattle lias his own form of poll-taking. McGaffin, a TV newsman, strolled into a Seattle restaurant recently, rapped on the table for attention and then rose with glass in hand. He announced in a booming voice: "A toast to the President of the United States." McGaffin was inimod iately showered with thrown bread, silverware and even a tortilla. Undaunted. he continued with his poll experiment in other restaurants: the results were always the same, although the food thrown at him varied from place to place. MeGaffin's analysis? "The President is in deep trouble."