[Hotword] Implement audio history pref accessing and setting.

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.
 {
   'variables': {
     'chrome_unit_tests_sources': [
       '../apps/saved_files_service_unittest.cc',
       '../components/autofill/content/renderer/test_password_autofill_agent.cc',
       '../components/autofill/content/renderer/test_password_autofill_agent.h',
       '../components/autofill/content/renderer/test_password_generation_agent.cc',
@@ skipping 461 matching lines @@
       'browser/history/shortcuts_database_unittest.cc',
       'browser/history/thumbnail_database_unittest.cc',
       'browser/history/top_sites_cache_unittest.cc',
       'browser/history/top_sites_database_unittest.cc',
       'browser/history/top_sites_impl_unittest.cc',
       'browser/history/typed_url_syncable_service_unittest.cc',
       'browser/history/url_utils_unittest.cc',
       'browser/history/visit_database_unittest.cc',
       'browser/history/visit_filter_unittest.cc',
       'browser/history/visit_tracker_unittest.cc',
+      'browser/history/web_history_service_unittest.cc',
       'browser/image_holder_unittest.cc',
       'browser/importer/firefox_profile_lock_unittest.cc',
       'browser/importer/profile_writer_unittest.cc',
       'browser/internal_auth_unittest.cc',
       'browser/install_verification/win/imported_module_verification_unittest.cc',
       'browser/install_verification/win/loaded_module_verification_unittest.cc',
       'browser/install_verification/win/loaded_modules_snapshot_unittest.cc',
       'browser/install_verification/win/module_ids_unittest.cc',
       'browser/install_verification/win/module_info_unittest.cc',
       'browser/install_verification/win/module_list_unittest.cc',
@@ skipping 2435 matching lines @@
               'dependencies': [
                 '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
               ],
             }],
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }