UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36

Steps to reproduce the problem:
The following example uses Google Play Music, but this issue affects any use of media keys by Chromium running under GNOME 3.

1. Install the Google Play Music extension: https://chrome.google.com/webstore/detail/google-play-music/fahmaaghhglfmonjliepjlchgpgfmobi
2. Open Google Play Music: https://play.google.com/music/listen
3. Begin listening to any station, playlist, artist, album, or song, or resume playback of the existing queue.
4. Using a keyboard that has media keys, press the "Play/Pause" key, "Next Track" key, or "Previous Track" key.

What is the expected behavior?
Google Play Music should react to the pressed key.

What went wrong?
A box with the 🚫 symbol is briefly overlaid on the screen by GNOME 3 instead.

Did this work before? No 

Chrome version: 63.0.3239.132  Channel: stable
OS Version: Fedora 27 Workstation x86_64
Flash Version: 

Under GNOME 3, media keys are captured by the gnome-settings-daemon process. Applications running under GNOME 3 which respond to media keys are expected to listen for signals through the D-Bus interface (org.gnome.SettingsDaemon.MediaKeys) instead of directly capturing the media keys.

The relevant D-Bus API is described here: https://github.com/GNOME/gnome-settings-daemon/tree/master/plugins/media-keys

Note that other code already exists in Chromium which detects the GNOME desktop environment and uses a specific D-Bus interface as a result. Please see:
src/services/device/wake_lock/power_save_blocker/power_save_blocker_x11.cc