Barclays Chief Executive C.S. Venkatakrishnan is just starting on the toughest part of his overhaul: Proving to shareholders that the lender can slug it out with American investment banks. “I’ve always said this is the hardest part of our story,” the Barclays CEO, who goes by Venkat, said in an interview in London. Barclays fleshed out details this week for how it plans to shift the focus of its investment-banking business.