One accessibility tree per frame.

chrome/browser/resources/chromeos/chromevox/cvox2/background/automation_predicate.js

 // Copyright 2014 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.
 
 /**
  * @fileoverview ChromeVox predicates for the automation extension API.
  */
 
 goog.provide('AutomationPredicate');
 goog.provide('AutomationPredicate.Binary');
@@ skipping 182 matching lines @@
   return fl.top != sl.top ||
       (fl.top + fl.height != sl.top + sl.height);
 };
 
 /**
  * Matches against a node that should be visited but not considered a leaf.
  * @param {!AutomationNode} node
  * @return {boolean}
  */
 AutomationPredicate.container = function(node) {
+  if (node.role == RoleType.rootWebArea)
+    return !node.parent || node.parent.root.role != RoleType.rootWebArea;
+
   return node.role == RoleType.toolbar ||
-      node.role == RoleType.rootWebArea ||
       node.role == RoleType.window;
 };
 
 /**
  * Leaf nodes that should be ignored while traversing the automation tree. For
  * example, apply this predicate when moving to the next element.
  * @param {!AutomationNode} node
  * @return {boolean}
  */
 AutomationPredicate.shouldIgnoreLeaf = function(node) {
   if (node.state.invisible ||
       (node.location.height == 0 && node.location.width == 0))
     return true;
 
   if (node.name || node.value)
     return false;
 
   return AutomationPredicate.leaf(node) &&
       (node.role == RoleType.client ||
        node.role == RoleType.div ||
        node.role == RoleType.group ||
        node.role == RoleType.image ||
        node.role == RoleType.staticText);
 };
 
 });  // goog.scope