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

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

Index: chrome/browser/resources/chromeos/switch_access/tree_walker.js
diff --git a/chrome/browser/resources/chromeos/switch_access/tree_walker.js b/chrome/browser/resources/chromeos/switch_access/tree_walker.js
new file mode 100644
index 0000000000000000000000000000000000000000..77b883a691fa5c69b42e0f9e96e3fae5f15c219f
--- /dev/null
+++ b/chrome/browser/resources/chromeos/switch_access/tree_walker.js
@@ -0,0 +1,240 @@
+// 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 move to the appropriate node in the accessibility tree.
+ *
+ * @constructor
+ */
+function AutomationTreeWalker() {};
+
+AutomationTreeWalker.prototype = {
+  /**
+   * Return the next interesting node after |node|. If no interesting node comes
+   * after |node|, return the first interesting node.
+   *
+   * @param {chrome.automation.AutomationNode} node
+   * @param {chrome.automation.AutomationNode} root
+   * @return {chrome.automation.AutomationNode}
+   */
+  moveToNext: function(node, root) {
+    if (node) {
+      let next = this.getNextNode_(node);

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

You could just have
do {
    node = this...
  while (...);

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

Done.

+      while (next && !this.isInteresting_(next))
+        next = this.getNextNode_(next);
+      if (next)
+        return next;
+    }
+
+    if (root) {
+      console.log('Reached the last interesting node. Restarting with first.');
+      let next = root.firstChild;
+      while (next && !this.isInteresting_(next))
+        next = this.getNextNode_(next);
+      if (next)
+        return next;
+    }
+
+    console.log('Found no interesting nodes to visit.');
+    return null;
+  },
+
+  /**
+   * Returns the previous interesting node before |node|. If no interesting node
+   * comes before |node|, return the last interesting node.
+   *
+   * @param {chrome.automation.AutomationNode} node
+   * @param {chrome.automation.AutomationNode} root
+   * @return {chrome.automation.AutomationNode}
+   */
+  moveToPrevious: function(node, root) {
+    if (node) {
+      let prev = this.getPreviousNode_(node);
+      while (prev && !this.isInteresting_(prev))
+        prev = this.getPreviousNode_(prev);
+      if (prev)
+        return prev;
+    }
+
+    if (root) {
+      console.log('Reached the first interesting node. Restarting with last.')
+      let prev = this.getYoungestDescendant_(root);
+      while (prev && !this.isInteresting_(prev))
+        prev = this.getPreviousNode_(prev);
+      if (prev)
+        return prev;
+    }

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

You can combine this function with the one above and take a direction param.

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

Done.

+
+    console.log('Found no interesting nodes to visit.')
+    return null;
+  },
+
+  /**
+   * Given a flat list of nodes in pre-order, get the node that comes after
+   * |node|.
+   *
+   * @param {!chrome.automation.AutomationNode} node
+   * @return {chrome.automation.AutomationNode}
+   * @private
+   */
+  getNextNode_: function(node) {
+    // Check for child.
+    let child = node.firstChild;
+    if (child)
+      return child;
+
+    // No child. Check for right-sibling.
+    let sibling = node.nextSibling;
+    if (sibling)
+      return sibling;
+
+    // No right-sibling. Get right-sibling of closest ancestor.
+    let ancestor = node.parent;
+    while (ancestor) {
+      let aunt = ancestor.nextSibling;
+      if (aunt)
+        return aunt;

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

Comment: very gender specific :).

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

Haha, no gender-neutral word for aunt or uncle yet (to my knowledge).

+      ancestor = ancestor.parent;
+    }
+
+    // No node found after |node|, so return null.
+    return null;
+  },
+
+  /**
+   * Given a flat list of nodes in pre-order, get the node that comes before
+   * |node|.
+   *
+   * @param {!chrome.automation.AutomationNode} node
+   * @return {chrome.automation.AutomationNode}
+   * @private
+   */
+  getPreviousNode_: function(node) {
+    // Check for left-sibling. Return its youngest descendant if it has one.
+    // Otherwise, return the sibling.
+    let sibling = node.previousSibling;
+    if (sibling) {
+      let descendant = this.getYoungestDescendant_(sibling);

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

return this.getYoungestDescendant_(sibling) || sibling;

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

Done.

+      if (descendant)
+        return descendant;
+      return sibling;
+    }
+
+    // No left-sibling. Check for parent.
+    let parent = node.parent;

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

return node.parent; (no need to do any of the below).

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

Done.

+    if (parent)
+      return parent;
+
+    // No node found before |node|, so return null.
+    return null;
+  },
+
+  /**
+   * Get the youngest descendant of |node| if it has one.
+   *
+   * @param {!chrome.automation.AutomationNode} node
+   * @return {chrome.automation.AutomationNode}
+   * @private
+   */
+  getYoungestDescendant_: function(node) {
+    if (!node.lastChild)
+      return null;
+
+    while (node.lastChild)
+      node = node.lastChild;
+
+    return node;
+  },
+
+  /**
+   * Returns true if |node| is interesting.
+   *
+   * @param {!chrome.automation.AutomationNode} node
+   * @return {boolean}
+   * @private
+   */
+  isInteresting_: function(node) {
+    return node.state && node.state.focusable;
+  },
+
+  /**
+   * Return the next sibling of |node| if it has one.
+   *
+   * @param {chrome.automation.AutomationNode} node
+   * @return {chrome.automation.AutomationNode}
+   */
+  debugMoveToNext: function(node) {
+    if (!node)
+      return null;
+
+    let next = node.nextSibling;
+    if (next) {
+      return next;
+    } else {
+      console.log('Node is last of siblings');
+      console.log('\n');
+      return null;
+    }
+  },
+
+  /**
+   * Return the previous sibling of |node| if it has one.
+   *
+   * @param {chrome.automation.AutomationNode} node
+   * @return {chrome.automation.AutomationNode}
+   */
+  debugMoveToPrevious: function(node) {
+    if (!node)
+      return null;
+
+    let prev = node.previousSibling;
+    if (prev) {
+      return prev;
+    } else {
+      console.log('Node is first of siblings');
+      console.log('\n');
+      return null;
+    }
+  },
+
+  /**
+   * Return the first child of |node| if it has one.
+   *
+   * @param {chrome.automation.AutomationNode} node
+   * @return {chrome.automation.AutomationNode}
+   */
+  debugMoveToFirstChild: function(node) {
+    if (!node)
+      return null;
+
+    let child = node.firstChild;
+    if (child) {
+      return child;
+    } else {
+      console.log('Node has no children');
+      console.log('\n');
+      return null;
+    }
+  },
+
+  /**
+   * Return the parent of |node| if it has one.
+   *
+   * @param {chrome.automation.AutomationNode} node
+   * @return {chrome.automation.AutomationNode}
+   */
+  debugMoveToParent: function(node) {
+    if (!node)
+      return null;
+
+    let parent = node.parent;
+    if (parent) {
+      return parent;
+    } else {
+      console.log('Node has no parent');
+      console.log('\n');
+      return null;
+    }
+  }
+};