Bug 1665496 - part1 : enable Media Session API by default. r=chunmin

We're going to enable MediaSession API by default on Fx82.

Differential Revision: https://phabricator.services.mozilla.com/D90476

modules/libpref/init/StaticPrefList.yaml

@@ -2138,7 +2138,7 @@
 # Media Session API
 - name: dom.media.mediasession.enabled
   type: bool
-  value: @IS_NIGHTLY_BUILD@
+  value: true
   mirror: always
 
 # Number of seconds of very quiet or silent audio before considering the audio