Issue 6775 - Work around filter data stored in IndexedDB getting lost on Microsoft Edge

lib/indexedDBBackup.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
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 "use strict";
 
 const {FilterNotifier} = require("../adblockpluscore/lib/filterNotifier");
 const {FilterStorage} = require("../adblockpluscore/lib/filterStorage");
 const {DownloadableSubscription, SpecialSubscription} =
   require("../adblockpluscore/lib/subscriptionClasses");
 
 const BACKUP_NAME = "file:indexedDB-backup";
 
-let minWriteInterval;

[Sebastian Noack, 2018/09/05 14:34:49]

Nit: This is no longer a "min" but a fixed value. Also "backupDelay" seems more
accurately describe it (than write interval).

[geo, 2018/09/05 15:59:08]

Done.
I've also changed the name of `writeInterval` variable in tests/indexedDBBackup
as this change made sense there as well.

+let backupDelay;
 let pendingBackup = false;
 
 function setBackupInterval(backupInterval = 60 * 1000)
 {
-  minWriteInterval = backupInterval;
+  backupDelay = backupInterval;
 }
 
 setBackupInterval();
 
 function scheduleBackup()
 {
   if (!pendingBackup)
   {
     pendingBackup = true;
 
     setTimeout(
       () =>
       {
         saveToStorage();
         pendingBackup = false;
       },
-      minWriteInterval
+      backupDelay
     );
   }
 }
 
 function saveToStorage()
 {
   browser.storage.local.set({
     [BACKUP_NAME]: {
       content: serialize(),
       lastModified: Date.now()
@@ skipping 50 matching lines @@
 FilterNotifier.on("filter.removed", scheduleBackup);
 FilterNotifier.on("filter.moved", scheduleBackup);
 FilterNotifier.on("filter.disabled", scheduleBackup);
 
 exports.IndexedDBBackup =
 {
   getBackupData,
   // Non-public API, just for tests.
   setBackupInterval
 };