WASHINGTON -— TikTok’s chief executive officer (CEO) said on Wednesday the social media company expects to win a legal challenge to block legislation signed into law by President Joseph R. Biden, Jr. that he said would ban its popular short video app used by 170 million Americans. “Rest assured — we aren’t going anywhere,” CEO Shou Zi Chew said in a video posted moments after Mr. Biden signed the bill that gives China-based ByteDance 270...