Add ToString to AXTreeUpdate.

ui/accessibility/ax_tree.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 "ui/accessibility/ax_tree.h"
 
 #include <set>
 
 #include "base/logging.h"
 #include "base/strings/stringprintf.h"
 #include "ui/accessibility/ax_node.h"
 
 namespace ui {
 
 namespace {
 
 std::string TreeToStringHelper(AXNode* node, int indent) {
-  std::string result;
-  for (int i = 0; i < indent; i++)
-    result += "  ";
+  std::string result = std::string(2 * indent, ' ');
   result += node->data().ToString() + "\n";
   for (int i = 0; i < node->child_count(); ++i)
     result += TreeToStringHelper(node->ChildAtIndex(i), indent + 1);
   return result;
 }
 
 }  // anonymous namespace
 
 // Intermediate state to keep track of during a tree update.
 struct AXTreeUpdateState {
@@ skipping 226 matching lines @@
       update_state->pending_nodes.insert(child);
       update_state->new_nodes.insert(child);
     }
     new_children->push_back(child);
   }
 
   return success;
 }
 
 }  // namespace ui