A man walks past a billboard advertisement for YouTube in Berlin, Germany. Sean Gallup/Getty Images Twitter recently announced that it is allowing YouTube videos to be directly embedded into posts. This signifies a shift in YouTube's video dominance, moving away from just a platform to a video-watching medium. It's a surprise this hasn't happened sooner. Chris Stokel-Walker is a freelance journalist and the author of "YouTubers: How YouTube shook up TV and created a new generation of stars," and the upcoming book "TikTok Boom: China, the US and the Superpower Race for Social Media." This is an opinion column. The