Add ToString to AXTreeUpdate.

ui/accessibility/ax_tree_update.h

 // 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.
 
 #ifndef UI_ACCESSIBILITY_AX_TREE_UPDATE_H_
 #define UI_ACCESSIBILITY_AX_TREE_UPDATE_H_
 
 #include <vector>
 
 #include "ui/accessibility/ax_node_data.h"
@@ skipping 32 matching lines @@
   // The id of a node to clear, before applying any updates,
   // or 0 if no nodes should be cleared. Clearing a node means deleting
   // all of its children and their descendants, but leaving that node in
   // the tree. It's an error to clear a node but not subsequently update it
   // as part of the tree update.
   int node_id_to_clear;
 
   // A vector of nodes to update, according to the rules above.
   std::vector<AXNodeData> nodes;
 
+  // Return a multi-line indented string representation, for logging.
+  std::string ToString() const;
+
   // TODO(dmazzoni): location changes
 };
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_TREE_UPDATE_H_