Make tree serialization more robust and add more unit tests.

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 29 matching lines @@
       'target_name': 'accessibility_unittests',
       'type': 'executable',
       'dependencies': [
         '../../base/base.gyp:base',
         '../../base/base.gyp:test_support_base',
         '../../testing/gtest.gyp:gtest',
         '../../testing/gtest.gyp:gtest_main',
         'accessibility',
       ],
       'sources': [
+        'ax_tree_serializer_unittest.cc',
         'ax_tree_unittest.cc',
       ]
     },
   ],
 }