Add ToString to AXTreeUpdate.

ui/accessibility/ax_tree_update.cc

Index: ui/accessibility/ax_tree_update.cc
diff --git a/ui/accessibility/ax_tree_update.cc b/ui/accessibility/ax_tree_update.cc
index dc785d9e06ee3d8947a1579f6860496690533dca..8e9d019349f3cb8fa72f86204f5cd5d15bd4da3d 100644
--- a/ui/accessibility/ax_tree_update.cc
+++ b/ui/accessibility/ax_tree_update.cc
@@ -4,6 +4,9 @@
 
 #include "ui/accessibility/ax_tree_update.h"
 
+#include "base/containers/hash_tables.h"
+#include "base/strings/string_number_conversions.h"
+
 namespace ui {
 
 AXTreeUpdate::AXTreeUpdate() : node_id_to_clear(0) {
@@ -12,4 +15,28 @@ AXTreeUpdate::AXTreeUpdate() : node_id_to_clear(0) {
 AXTreeUpdate::~AXTreeUpdate() {
 }
 
+std::string AXTreeUpdate::ToString() const {
+  std::string result;
+  if (node_id_to_clear != 0) {
+    result += "AXTreeUpdate: clear node " +
+        base::IntToString(node_id_to_clear) + "\n";
+  }
+
+  // The challenge here is that we want to indent the nodes being updated
+  // so that parent/child relationships are clear, but we don't have access
+  // to the rest of the tree for context, so we have to try to show the
+  // relative indentation of child nodes in this update relative to their
+  // parents.
+  base::hash_map<int32, int> id_to_indentation;
+  for (size_t i = 0; i < nodes.size(); ++i) {
+    int indent = id_to_indentation[nodes[i].id];
+    result += std::string(2 * indent, ' ');
+    result += nodes[i].ToString() + "\n";
+    for (size_t j = 0; j < nodes[i].child_ids.size(); ++j)
+      id_to_indentation[nodes[i].child_ids[j]] = indent + 1;
+  }
+
+  return result;
+}
+
 }  // namespace ui