WASHINGTON—Lina Khan, an antitrust researcher focused on Big Tech’s immense market power, was sworn in on Tuesday as chair of the U.S. Federal Trade Commission, a victory for progressives seeking a clampdown on tech firms who hold a hefty share of a growing sector of the economy. Hours earlier, the U.S. Senate had confirmed Khan, with bipartisan support. She recently taught at Columbia Law School. Previously, as a staffer for the House Judiciary Committee’s antitrust panel, she helped write a massive report alleging abuses of market dominance by Amazon.com Inc, Apple Inc, Facebook Inc, and Google parent Alphabet Inc. “We applaud President Biden and the Senate for recognizing the urgent need to address runaway corporate power,” advocacy group Public Citizen said in a statement. Sen. Elizabeth Warren (D-Mass.) tweeted that the administration’s selection of Khan was “tremendous news.” “With Chair Khan at the helm, we have a huge opportunity to …