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

chrome/browser/resources/chromeos/switch_access/auto_scan_manager.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.
+
+/**
+ * Class to handle changes to auto-scan.
+ *
+ * @constructor
+ * @param {SwitchAccessInterface} switchAccess
+ */
+function AutoScanManager(switchAccess) {
+  /**
+   * SwitchAccess reference.
+   * @private {SwitchAccessInterface}
+   */
+  this.switchAccess_ = switchAccess;
+
+  /**
+   * Auto-scan interval ID.
+   *
+   * @private {number}
+   */
+  this.intervalID_ = 0;
+
+  /**
+   * Length of auto-scan interval in seconds.
+   *
+   * @private {number}
+   */
+  this.scanTime_ =
+      /** @type {number} */ (switchAccess.switchAccessPrefs
+          .getPref('autoScanTime'));

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

Optional: you could get more type safety if you added methods like
getNumberPref, etc. This could avoid the cast and throw if the pref isn't found.

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

Done.

+
+  let enabled = switchAccess.switchAccessPrefs.getPref('enableAutoScan');
+  if (enabled)
+    this.start_();
+};
+
+AutoScanManager.prototype = {
+  /**
+   * Stop the window from moving to the next node at a fixed interval.
+   *
+   * @private
+   */
+  stop_: function() {
+    window.clearInterval(this.intervalID_);
+    this.intervalID_ = 0;

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

Would be clearer to make this undefined. setInterval doesn't appear to return id
0, but I don't see why it couldn't.

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

Done. I think the documentation said that the interval ID would always be
non-zero, but I agree that undefined is probably clearer anyways.

+  },
+
+  /**
+   * Set the window to move to the next node at an interval in seconds equal to
+   * this.scanTime_.
+   *
+   * @private
+   */
+  start_: function() {
+    this.intervalID_ = window.setInterval(
+        this.switchAccess_.moveToNext.bind(this.switchAccess_),
+        this.scanTime_ * 1000);

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

Any reason to not just store the scan time in ms?

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

Done. Didn't have a reason besides that the options page displays the time in
seconds.

+  },
+
+  /**
+   * Return true if auto-scan is currently running. Otherwise return false.
+   *
+   * @return {boolean}
+   */
+  isRunning: function() {
+    return this.intervalID_ !== 0;
+  },
+
+  /**
+   * Restart auto-scan under the current settings.
+   */
+  restart: function() {
+    this.stop_();
+    this.start_();
+  },
+
+  /**
+   * Stop auto-scan if it is currently running. Then, if |enabled| is true,
+   * turn on auto-scan. Otherwise leave it off.
+   *
+   * @param {boolean} enabled
+   */
+  setEnabled: function(enabled) {
+    if (this.isRunning())
+      this.stop_();
+    if (enabled)
+      this.start_();
+  },
+
+  /**
+   * Update this.scanTime_ to |scanTime|. Then, if auto-scan is currently
+   * running, restart it.
+   *
+   * @param {number} scanTime Auto-scan interval time in seconds.
+   */
+  setScanTime: function(scanTime) {
+    this.scanTime_ = scanTime;
+    if (this.isRunning()) {
+      this.restart();
+    }
+  },
+};