Add AXTreeDelegate and refactor other AXTree classes slightly.

ui/accessibility/accessibility.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
 
   'targets': [
@@ skipping 25 matching lines @@
         'ax_tree_update.cc',
         'ax_tree_update.h',
       ]
     },
     {
       'target_name': 'accessibility_unittests',
       'type': 'executable',
       'dependencies': [
         '../../base/base.gyp:base',
         '../../base/base.gyp:run_all_unittests',
+        '../../testing/gmock.gyp:gmock',

[tfarina, 2014/01/11 23:58:51]

no, no. Do not use Gmock. Ben banned it!

         '../../testing/gtest.gyp:gtest',
         '../gfx/gfx.gyp:gfx',
         '../gfx/gfx.gyp:gfx_geometry',
         'accessibility',
       ],
       'sources': [
         'ax_generated_tree_unittest.cc',
         'ax_tree_serializer_unittest.cc',
         'ax_tree_unittest.cc',
       ]
     },
   ],
 }