I am being driven crazy by the fact that my media keys aren't working for iTunes. Such a pain! How can I fix this? I've tried the whole Chrome Extensions thing, that being I deleted all my extensions. Still, nothing has fixed this. Thanks to all who reply!
This is an intentional change in macOS High Sierra. Rather than the media keys always controlling iTunes, they will now control whatever media is playing at a given moment.
9To5Mac has detailed a way to change this behavior with a third-party application, macOS High Sierra Media Key Enabler for iTunes and Spotify:
Apple just released High Sierra and it brought good things and annoying things : they changed the behaviour of the media controller keys, they no longer control itunes, they control the video playback in safari. This pissed off a lot of people including me, so I just created a menu bar app to proxy media key events to iTunes/Spotify while Apple fixes this. It doesn't support touchbar yet, only physical buttons.