An accessibility tree update with two roots should be rejected.

ui/accessibility/ax_tree_unittest.cc

Index: ui/accessibility/ax_tree_unittest.cc
diff --git a/ui/accessibility/ax_tree_unittest.cc b/ui/accessibility/ax_tree_unittest.cc
index 83a8ae5abd5aeb86a3b1001f247cd7998e9d61fe..98ca7e5b2c896702920ac9da200472459f37a345 100644
--- a/ui/accessibility/ax_tree_unittest.cc
+++ b/ui/accessibility/ax_tree_unittest.cc
@@ -283,6 +283,24 @@ TEST(AXTreeTest, InvalidReparentingFails) {
   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);