Remove the last usages of PrefObserver outside of Prefs.

chrome/chrome_tests_unit.gypi

 # Copyright 2012 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.
 {
   'targets': [
     {
       # This target contains mocks and test utilities that don't belong in
       # production libraries but are used by more than one test executable.
       'target_name': 'test_support_common',
       'type': 'static_library',
@@ skipping 125 matching lines @@
         'browser/password_manager/mock_password_store.cc',
         'browser/password_manager/mock_password_store.h',
         'browser/password_manager/password_form_data.cc',
         'browser/password_manager/password_form_data.h',
         'browser/policy/mock_cloud_policy_data_store.cc',
         'browser/policy/mock_cloud_policy_data_store.h',
         'browser/policy/mock_configuration_policy_provider.cc',
         'browser/policy/mock_configuration_policy_provider.h',
         'browser/policy/policy_builder.cc',
         'browser/policy/policy_builder.h',
-        'browser/prefs/pref_observer_mock.cc',
-        'browser/prefs/pref_observer_mock.h',
+        'browser/prefs/mock_pref_change_callback.cc',
+        'browser/prefs/mock_pref_change_callback.h',
         'browser/prefs/pref_service_mock_builder.cc',
         'browser/prefs/pref_service_mock_builder.h',
         'browser/printing/print_preview_test.cc',
         'browser/printing/print_preview_test.h',
         'browser/protector/mock_protector_service.cc',
         'browser/protector/mock_protector_service.h',
         'browser/protector/mock_setting_change.cc',
         'browser/protector/mock_setting_change.h',
         'browser/search_engines/template_url_service_test_util.cc',
         'browser/search_engines/template_url_service_test_util.h',
@@ skipping 2137 matching lines @@
             'unit_tests.isolate',
           ],
           'sources': [
             'unit_tests.isolate',
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }