An accessibility tree update with two roots should be rejected.

ui/accessibility/ax_tree_unittest.cc

 // Copyright 2013 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 "base/memory/scoped_ptr.h"
 #include "base/strings/string_number_conversions.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/accessibility/ax_node.h"
 #include "ui/accessibility/ax_serializable_tree.h"
 #include "ui/accessibility/ax_tree.h"
@@ skipping 265 matching lines @@
   update.nodes[0].id = 1;
   update.nodes[0].role = AX_ROLE_ROOT_WEB_AREA;
   update.nodes[0].child_ids.push_back(3);
   update.nodes[0].child_ids.push_back(2);
   update.nodes[1].id = 2;
   update.nodes[2].id = 3;
   EXPECT_FALSE(tree.Unserialize(update));
   ASSERT_EQ("Node 3 reparented from 2 to 1", tree.error());
 }
 
+TEST(AXTreeTest, TwoRootsFails) {
+  AXTreeUpdate initial_state;
+  initial_state.nodes.resize(1);
+  initial_state.nodes[0].id = 1;
+  initial_state.nodes[0].role = AX_ROLE_ROOT_WEB_AREA;
+  AXTree tree(initial_state);
+
+  // This should fail because there are two new roots.
+  AXTreeUpdate update;
+  update.nodes.resize(2);
+  update.nodes[0].id = 2;
+  update.nodes[0].role = AX_ROLE_ROOT_WEB_AREA;
+  update.nodes[1].id = 3;
+  update.nodes[1].role = AX_ROLE_ROOT_WEB_AREA;
+  EXPECT_FALSE(tree.Unserialize(update));
+  ASSERT_EQ("Tree update contains two new roots", tree.error());
+}
+
 TEST(AXTreeTest, TreeDelegateIsCalled) {
   AXTreeUpdate initial_state;
   initial_state.nodes.resize(2);
   initial_state.nodes[0].id = 1;
   initial_state.nodes[0].role = AX_ROLE_ROOT_WEB_AREA;
   initial_state.nodes[0].child_ids.push_back(2);
   initial_state.nodes[1].id = 2;
 
   AXTree tree(initial_state);
   AXTreeUpdate update;
@@ skipping 25 matching lines @@
 
   ASSERT_EQ(1U, fake_delegate.node_creation_finished_ids().size());
   EXPECT_EQ(4, fake_delegate.node_creation_finished_ids()[0]);
 
   ASSERT_EQ(true, fake_delegate.root_changed());
 
   tree.SetDelegate(NULL);
 }
 
 }  // namespace ui