Add bug number to comment about implementing notifications.onClose.byUser support. (no bug) r=trivial DONTBUILD

MozReview-Commit-ID: HFernhOu4lG

--HG--
extra : rebase_source : eacece7d19a421a270fbbcbecae816c0c4ff4d68

toolkit/components/extensions/ext-notifications.js

@@ -142,7 +142,7 @@ this.notifications = class extends ExtensionAPI {
 
         onClosed: new EventManager(context, "notifications.onClosed", fire => {
           let listener = (event, notificationId) => {
-            // FIXME: Support the byUser argument.
+            // FIXME: Support the byUser argument (bug 1413188).
             fire.async(notificationId, true);
           };