Improve AXTree unit tests.

ui/accessibility/ax_tree.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_H_
 #define UI_ACCESSIBILITY_AX_TREE_H_
 
 #include <set>
 
 #include "base/containers/hash_tables.h"
@@ skipping 90 matching lines @@
   // should not be trusted any longer.
   virtual bool Unserialize(const AXTreeUpdate& update);
 
   // Return a multi-line indented string representation, for logging.
   std::string ToString() const;
 
   // A string describing the error from an unsuccessful Unserialize,
   // for testing and debugging.
   const std::string& error() const { return error_; }
 
+  int size() { return static_cast<int>(id_map_.size()); }
+
  private:
   AXNode* CreateNode(AXNode* parent, int32 id, int32 index_in_parent);
 
   // This is called from within Unserialize(), it returns true on success.
   bool UpdateNode(const AXNodeData& src, AXTreeUpdateState* update_state);
 
   void OnRootChanged();
 
   // Notify the delegate that the subtree rooted at |node| will be destroyed,
   // then call DestroyNodeAndSubtree on it.
@@ skipping 21 matching lines @@
 
   AXTreeDelegate* delegate_;
   AXNode* root_;
   base::hash_map<int32, AXNode*> id_map_;
   std::string error_;
 };
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_TREE_H_