Sen. Chuck Grassley, the Senate president pro tem, is the third in line for the presidency. Craig Hudson/Sipa USA via APThe presidential line of succession isn’t top of mind for most senators, but Saturday’s shooting at the White House Correspondents’ Association dinner changed that.Sen. Sheldon Whitehouse, who was in the ballroom, said about the rapid law-enforcement response pulling Cabinet members and top congressional leaders: “They were def…