extensionSystem: Fix updating extension list on session update

Before reenabling all extensions, we update the list of enabled extensions
to catch any changes that happened while extensions were enabled. However
this is currently broken as onEnabledExtensionsChanged() is a nop while
disabled, so just call getEnabledExtensions() directly.

js/ui/extensionSystem.js

@@ -334,7 +334,7 @@ function _sessionUpdated() {
     // from allowExtensions in the future
     if (Main.sessionMode.allowExtensions) {
         if (initted)
-            onEnabledExtensionsChanged();
+            enabledExtensions = getEnabledExtensions();
         enableAllExtensions();
     } else {
         disableAllExtensions();