Add ToString to AXTreeUpdate.

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 f4c609ed36b4cea2909302ada79aead060bc3aac..0001474577d1a873a8048670eaa6b1b73e103517 100644
--- a/ui/accessibility/ax_tree_unittest.cc
+++ b/ui/accessibility/ax_tree_unittest.cc
@@ -103,6 +103,44 @@ TEST(AXTreeTest, SerializeSimpleAXTree) {
       dst_tree.ToString());
 }
 
+TEST(AXTreeTest, SerializeAXTreeUpdate) {
+  AXNodeData list;
+  list.id = 3;
+  list.role = AX_ROLE_LIST;
+  list.state = 0;
+  list.child_ids.push_back(4);
+  list.child_ids.push_back(5);
+  list.child_ids.push_back(6);
+
+  AXNodeData list_item_2;
+  list_item_2.id = 5;
+  list_item_2.role = AX_ROLE_LIST_ITEM;
+  list_item_2.state = 0;
+
+  AXNodeData list_item_3;
+  list_item_3.id = 6;
+  list_item_3.role = AX_ROLE_LIST_ITEM;
+  list_item_3.state = 0;
+
+  AXNodeData button;
+  button.id = 7;
+  button.role = AX_ROLE_BUTTON;
+  button.state = 0;
+
+  AXTreeUpdate update;
+  update.nodes.push_back(list);
+  update.nodes.push_back(list_item_2);
+  update.nodes.push_back(list_item_3);
+  update.nodes.push_back(button);
+
+  EXPECT_EQ(
+      "id=3 list (0, 0)-(0, 0) child_ids=4,5,6\n"
+      "  id=5 list_item (0, 0)-(0, 0)\n"
+      "  id=6 list_item (0, 0)-(0, 0)\n"
+      "id=7 button (0, 0)-(0, 0)\n",
+      update.ToString());
+}
+
 TEST(AXTreeTest, DeleteUnknownSubtreeFails) {
   AXNodeData root;
   root.id = 1;