One accessibility tree per frame.

chrome/browser/resources/chromeos/chromevox/cvox2/background/background_test.extjs

 // 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.
 
 // Include test fixture.
 GEN_INCLUDE(['../../testing/chromevox_next_e2e_test_base.js',
              '../../testing/assert_additions.js']);
 
 GEN_INCLUDE(['../../testing/mock_feedback.js']);
 
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     <h2>foxtraut</h2>
     <p>end<span>of test</span></p>
   */},
 
   formsDoc: function() {/*!
     <select id="fruitSelect">
       <option>apple</option>
       <option>grape</option>
       <option> banana</option>
     </select>
-  */}
+  */},
+
+  iframesDoc: function() {/*!
+    <p>start</p>
+    <button>Before</button>
+    <iframe srcdoc="<button>Inside</button>"></iframe>
+    <button>After</button>
+  */},
 };
 
 /** Tests that ChromeVox classic is in this context. */
 SYNC_TEST_F('BackgroundTest', 'ClassicNamespaces', function() {
   assertEquals('object', typeof(cvox));
   assertEquals('function', typeof(cvox.ChromeVoxBackground));
 });
 
 /** Tests that ChromeVox next is in this context. */
 SYNC_TEST_F('BackgroundTest', 'NextNamespaces', function() {
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     var focusButton = go.focus.bind(go);
     var focusSlider = slider.focus.bind(slider);
     mockFeedback.call(focusButton)
         .expectNextSpeechUtteranceIsNot('noisy')
         .call(focusSlider)
         .expectSpeech('noisy')
         .expectSpeech('noisy')
         .replay();
   }.bind(this));
 });
+
+/** Tests navigating into and out of iframes.. */
+TEST_F('BackgroundTest', 'ForwardNavigationThroughIframes', function() {
+  var mockFeedback = this.createMockFeedback();
+
+  var runTestIfIframeIsLoaded = function(rootNode) {
+    // Return if the iframe hasn't loaded yet.
+    var iframe = rootNode.find({role: 'iframe'});
+    var childDoc = iframe.firstChild;
+    if (childDoc && childDoc.children.length == 0) {
+      return;
+    }
+
+    var doCmd = this.doCmd.bind(this);
+
+    mockFeedback.expectSpeech('start').expectBraille('start');
+
+    mockFeedback.call(doCmd('nextButton'))
+        .expectSpeech('Before', 'Button');
+    mockFeedback.call(doCmd('nextButton'))
+        .expectSpeech('Inside', 'Button');
+    mockFeedback.call(doCmd('nextButton'))
+        .expectSpeech('After', 'Button');
+
+    mockFeedback.replay();
+  }.bind(this);
+
+  this.runWithLoadedTree(this.iframesDoc, function(rootNode) {
+    chrome.automation.getDesktop(function(desktopNode) {
+      runTestIfIframeIsLoaded(rootNode);
+
+      desktopNode.addEventListener('loadComplete', function(evt) {
+        runTestIfIframeIsLoaded(rootNode);
+      }, true);      
+    });
+  });
+});