Bug 1497983 - Allow pref to be read of the main thread. r=dminor

Differential Revision: https://phabricator.services.mozilla.com/D8359

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

modules/libpref/init/StaticPrefList.h

@@ -1123,7 +1123,7 @@ VARCACHE_PREF(
 VARCACHE_PREF(
   "media.ffmpeg.low-latency.enabled",
    MediaFfmpegLowLatencyEnabled,
-  bool, false
+  RelaxedAtomicBool, false
 )
 #endif