ByteDance, the operator of the TikTok video app, said its popular video app in China, called Douyin, attracted more than 400 million daily active users as of Jan. 5, compared to 250 million a year earlier.  Douyin’s ability to keep more users hooked is crucial to ByteDance’s ability to keep increasing its revenue in 2020. Even though the rapid expansion of TikTok, which doesn’t operate in China,...