Added auto-scan, made some small changes to how prefs are stored, refactored.

chrome/browser/resources/chromeos/switch_access/testable_switch_access.js

-// Copyright 2017 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-/**
- * Subclass of SwitchAccess that exposes SwitchAccess's methods for testing.
- */
-let TestableSwitchAccess = function() {
-  SwitchAccess.call(this);
-};
-
-TestableSwitchAccess.prototype = {
-  __proto__: SwitchAccess.prototype,
-
-  init_: function() {},
-
-  getNextNode: function(node) {
-    return this.getNextNode_(node);
-  },
-
-  getPreviousNode: function(node) {
-    return this.getPreviousNode_(node);
-  },
-
-  getYoungestDescendant: function(node) {
-    return this.getYoungestDescendant_(node);
-  },
-
-  isInteresting: function(node) {
-    return this.isInteresting_(node);
-  }
-};