Bug 1606588 - Enable media control key event on OSX version Nightly. r=chunmin

Differential Revision: https://phabricator.services.mozilla.com/D58497

--HG--
extra : moz-landing-system : lando

modules/libpref/init/StaticPrefList.yaml

@@ -6544,7 +6544,7 @@
 
 - name: media.hardwaremediakeys.enabled
   type: bool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 - name: media.webrtc.platformencoder