I wanted to use the media keys on my laptop to control YouTube from any tab. There wasn't any good solution (that I could find) so I built my own. I made a Chrome Extension that controls all open YouTube tabs/windows and allows control from any tab/window in Chrome. The Play/Pause key on your keyboard works as expected and the Previous and Next keys go to the next or previous video in a YouTube playlist!
Try it out by downloading it on GitHub.