Viola Davis and Justin Trudeau are among the people who shared a viral mistruth about Iranian protesters in the past several days. A false claim that Iran plans to execute thousands of people has gone viral in the wake of the first death sentence for a protester tied to the demonstrations against the country’s clerical rulers over women’s rights. An image that has circulated widely on social media falsely says 15,000 protesters have been sentenced to death. That claim is not true, but it has been amplified by major public figures, including the actors Viola Davis and Sophie Turner and...