This implements CanSetFullscreen to return true when it's a video and lets the Fullscreen property reflect and change the fullscreen state of the player.

BUG: 408059

YouTube sets the entire website as fullscreen and gets confused when just fullscreening the player but it works fine in e.g. Vimeo.
If the video autostarted it refuses to fullscreen it as there hasn't been any user interaction, otherwise works fine.