One of the headline features of Android O is picture-in-picture video support across all devices. The feature has been part of Android TV since Nougat, but starting with O, it works on phones and tablets too. A handful of applications have already added support for the feature, including VLC, Duo, and Play Movies - now it's Chrome's turn.
If you're on Android O, just go full-screen on any video in Chrome and tap the Recents button twice. Pretty simple. I don't have a device running Android O to verify what branches of Chrome currently have this feature, but tipster Nick was using Chrome Dev.
Let us know in the comments if you're on Android O and if this is working for you. Now I just need Plex to add support...