Bug 1544562 - Call onUnload when about:preferences is closed r=rstrong

Differential Revision: https://phabricator.services.mozilla.com/D27591

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

browser/components/preferences/in-content/main.js

@@ -1436,6 +1436,9 @@ var gMainPane = {
   handleEvent(aEvent) {
     if (aEvent.type == "unload") {
       this.destroy();
+      if (AppConstants.MOZ_UPDATER) {
+        onUnload();
+      }
     }
   },