Bug 1741677 - Make media.flac.enabled pref into a RelaxedAtomicBool. r=alwu

Depends on D131431

Differential Revision: https://phabricator.services.mozilla.com/D131432

modules/libpref/init/StaticPrefList.yaml

@@ -8749,7 +8749,7 @@
   mirror: always
 
 - name: media.flac.enabled
-  type: bool
+  type: RelaxedAtomicBool
   value: true
   mirror: always