One accessibility tree per frame.

chrome/test/data/extensions/api_test/automation/tests/desktop/focus_iframe.js

+// Copyright 2016 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.
+
+var allTests = [
+  function testFocusInIframes() {
+    chrome.test.getConfig(function(config) {
+      var url = 'http://a.com:' + config.testServer.port + '/iframe_outer.html';
+      chrome.tabs.create({url: url});
+
+      chrome.automation.getDesktop(function(rootNode) {
+        // Succeed when the button inside the iframe gets focus.
+        rootNode.addEventListener('focus', function(event) {
+          if (event.target.name == 'Inner')
+            chrome.test.succeed();
+        });
+
+        // Wait for the inner frame to load, then find the button inside it
+        // and focus it.
+        rootNode.addEventListener('loadComplete', function(event) {
+          if (event.target.url.indexOf('iframe_inner.html') >= 0) {
+            chrome.automation.getFocus(function(focus) {
+              // Assert that the outer frame has focus.
+              assertTrue(focus.url.indexOf('iframe_outer') >= 0);
+
+              // Find the inner button and focus it.
+              var innerButton = focus.find({ attributes: { name: 'Inner' } });
+              innerButton.focus();
+            });
+          }
+        });
+      });
+    });
+  },
+];
+
+chrome.test.runTests(allTests);