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

chrome/browser/resources/chromeos/switch_access/testable_tree_walker.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.
+ * Subclass of AutomationTreeWalker that exposes AutomationTreeWalker's methods
+ * for testing.

[David Tseng, 2017/03/29 21:57:20]

@extends {AutomationTreeWalker}
(I know this doesn't get compiled).

[elichtenberg, 2017/03/30 21:46:56]

Done.

  */
-let TestableSwitchAccess = function() {
-  SwitchAccess.call(this);
+function TestableTreeWalker() {
+  AutomationTreeWalker.call(this);
 };
 
-TestableSwitchAccess.prototype = {
-  __proto__: SwitchAccess.prototype,
-
-  init_: function() {},
+TestableTreeWalker.prototype = {
+  __proto__: AutomationTreeWalker.prototype,
 
   getNextNode: function(node) {

[David Tseng, 2017/03/29 21:57:21]

I don't see that much value in having this in its own file since it has one
consumer. Up to you though. I would put it in the unit test file. Also, since
tests don't get compiled, you could access private members of the object.

[elichtenberg, 2017/03/30 21:46:56]

I agree. I'm just going to access the private members of AutomationTreeWalker,
and so I'm deleting this file.

     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);
   }
 };