Burger King, which is majority-owned by 3G Capital, would create the world’s third-largest fast-food chain by merging with Canada’s bigger seller of coffee and doughnuts, the companies said in a statement. Canada’s corporate tax rate is 26.5 percent, compared with 40 percent in the U.S., according to audit, tax and advisory firm KPMG’s website. The deal threatens to renew debate over American companies shifting their headquarters internationally in search of a lower corporate tax bill.