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

chrome/browser/resources/chromeos/switch_access/options.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.
 
 /**
  * Alias for document.getElementById.
  *
  * @param  {string} id
  * @return {Element}
  */
@@ skipping 23 matching lines @@
 };
 
 SwitchAccessOptions.prototype = {
   /**
    * Initialize the options page by setting all elements representing a user
    * preference to show the correct value.
    *
    * @private
    */
   init_: function() {
-    let prefs = this.switchAccessPrefs_.getPrefs();
-    $('enableAutoScan').checked = prefs['enableAutoScan'];
-    $('autoScanTime').value = prefs['autoScanTime'];
+    $('enableAutoScan').checked =
+        this.switchAccessPrefs_.getPref('enableAutoScan');
+    $('autoScanTime').value = this.switchAccessPrefs_.getPref('autoScanTime');
   },
 
   /**
    * Handle a change by the user to an element representing a user preference.
    *
    * @param {!Event} event
    * @private
    */
   handleInputChange_: function(event) {
-    switch (event.target.id) {
+    let input = event.target;
+    switch (input.id) {
       case 'enableAutoScan':
-        this.switchAccessPrefs_.setPref(event.target.id, event.target.checked);
+        this.switchAccessPrefs_.setPref(input.id, input.checked);
         break;
       case 'autoScanTime':
-        this.switchAccessPrefs_.setPref(event.target.id, event.target.value);
+        let oldVal =
+            /** @type {number} */ (this.switchAccessPrefs_.getPref(input.id));
+        let val = Number(input.value);
+        let min = Number(input.min);
+        if (this.isValidInput_(val, oldVal, min)) {
+          input.value = val;
+          this.switchAccessPrefs_.setPref(input.id, val);
+        } else {
+          input.value = oldVal;
+        }
         break;
     }
   },
 
   /**
+   * Return true if the input is a valid autoScanTime input. Otherwise, return
+   * false.
+   *
+   * @param {number} value
+   * @param {number} oldValue
+   * @param {number} min
+   * @return {boolean}
+   */
+  isValidInput_: function(value, oldValue, min) {
+    return (value !== oldValue) && (value >= min);
+  },
+
+  /**
    * Handle a change in user preferences.
    *
    * @param {!Event} event
    * @private
    */
   handlePrefsUpdate_: function(event) {
     let updatedPrefs = event.detail;
     for (let key of Object.keys(updatedPrefs)) {
       switch (key) {
         case 'enableAutoScan':
           $(key).checked = updatedPrefs[key];
           break;
         case 'autoScanTime':
           $(key).value = updatedPrefs[key];
           break;
       }
     }
   }
 };
 
 document.addEventListener('DOMContentLoaded', function() {
   new SwitchAccessOptions();
 });