Issue 6908 - Pass updated subscription's old filters as an event argument

lib/filterStorage.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 179 matching lines @@
   },
 
   /**
    * Replaces the list of filters in a subscription by a new list
    * @param {Subscription} subscription filter subscription to be updated
    * @param {Filter[]} filters new filter list
    */
   updateSubscriptionFilters(subscription, filters)
   {
     removeSubscriptionFilters(subscription);
-    subscription.oldFilters = subscription.filters;
+    let oldFilters = subscription.filters;
     subscription.filters = filters;
     addSubscriptionFilters(subscription);
-    filterNotifier.emit("subscription.updated", subscription);
-    delete subscription.oldFilters;
+    filterNotifier.emit("subscription.updated", subscription, oldFilters);
   },
 
   /**
    * Adds a user-defined filter to the list
    * @param {Filter} filter
    * @param {SpecialSubscription} [subscription]
    *   particular group that the filter should be added to
    * @param {number} [position]
    *   position within the subscription at which the filter should be added
    */
@@ skipping 457 matching lines @@
  * @param {Subscription} subscription filter subscription to be removed
  */
 function removeSubscriptionFilters(subscription)
 {
   if (!FilterStorage.knownSubscriptions.has(subscription.url))
     return;
 
   for (let filter of subscription.filters)
     filter.subscriptions.delete(subscription);
 }