Bug 1874738 - Unship non-standard mozStartedLoadingTextTrack event r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D198648

modules/libpref/init/StaticPrefList.yaml

@@ -10357,7 +10357,7 @@
 # This pref controls whether dispatch testing-only events.
 - name: media.webvtt.testing.events
   type: bool
-  value: true
+  value: false
   mirror: always
 
 - name: media.webspeech.synth.force_global_queue