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 173 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.triggerListeners("subscription.updated", subscription);
-    delete subscription.oldFilters;
+    FilterNotifier.triggerListeners("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 564 matching lines @@
       else if (this.wantObj === false && val)
         this.curObj.push(val.replace(/\\\[/g, "["));
     }
     finally
     {
       Filter.knownFilters = origKnownFilters;
       Subscription.knownSubscriptions = origKnownSubscriptions;
     }
   }
 };