message: Detect support for mediaSession
  Firefox doesn't support mediaSession so don't try setting handlers
diff: @@ -498,6 +498,7 @@ if (!global.mxCallHandler) {
  // add empty handlers for media actions, otherwise the media keys
  // end up causing the audio elements with our ring/ringback etc
  // audio clips in to play.
  + if (navigator.mediaSession) {
  navigator.mediaSession.setActionHandler('play', function() {});
  navigator.mediaSession.setActionHandler('pause', function() {});
  navigator.mediaSession.setActionHandler('seekbackward', function() {});
  @@ -505,6 +506,7 @@ if (!global.mxCallHandler) {
  navigator.mediaSession.setActionHandler('previoustrack', function() {});
  navigator.mediaSession.setActionHandler('nexttrack', function() {});
  }
  +}
  const callHandler = {
  getCallForRoom: function(roomId) {
  