Added unit tests for tree_walker and automation_predicate.

chrome/browser/resources/chromeos/switch_access/automation_predicate.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 containing predicates for the chrome automation API. Each predicate
  * can be run on one or more AutomationNodes and returns a boolean value.
  *
  * @constructor
  */
@@ skipping 29 matching lines @@
     return false;
 
   // Work around for client nested in client. No need to have user select both
   // clients for a window. Once locations for outer client updates correctly,
   // this won't be needed.
   if (node.role === chrome.automation.RoleType.CLIENT
       && node.role === scope.role && node !== scope)
     return false;
 
   let interestingBranches = 0;
-  for (let child of node.children) {
+  let children = node.children || [];
+  for (let child of children) {
     if (AutomationPredicate.isInterestingSubtree(child))
       interestingBranches += 1;
     if (interestingBranches > 1)
       return true;
   }
   return false;
 };
 
 /**
  * Returns true if the two nodes have the same location.
@@ skipping 54 matching lines @@
     // Work around for browser tabs
     if (role === chrome.automation.RoleType.TAB
         && parent.role === chrome.automation.RoleType.TAB_LIST
         && root.role === chrome.automation.RoleType.DESKTOP)
       return true;
   }
 
   // The general rule that applies to everything.
   return state[chrome.automation.StateType.FOCUSABLE] === true;
 }