Kirstjen Nielsen, the homeland security secretary, told colleagues she was close to resigning after President Trump berated her on Wednesday in front of the entire cabinet for what he said was her failure to adequately secure the nation’s borders, according to several current and former officials familiar with the episode. Ms. Nielsen, who is a protégée of John F. Kelly, the White House chief of staff, has drafted a resignation letter but has not submitted it, according to two of the people. As the head of the Department of Homeland Security, Ms. Nielsen is in charge of the 20,000 employees...