The chrome.screenlockPrivate API allows select apps to control the ChromeOS ScreenLocker.

chrome/chrome_tests.gypi

 # 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' : {
     'chromedriver_unittest_sources': [
       'test/chromedriver/capabilities_unittest.cc',
       'test/chromedriver/chrome/chrome_finder_unittest.cc',
       'test/chromedriver/chrome/console_logger_unittest.cc',
       'test/chromedriver/chrome/device_manager_unittest.cc',
@@ skipping 991 matching lines @@
         'browser/chrome_security_exploit_browsertest.cc',
         'browser/chrome_switches_browsertest.cc',
         'browser/chromeos/accessibility/accessibility_manager_browsertest.cc',
         'browser/chromeos/accessibility/magnification_manager_browsertest.cc',
         'browser/chromeos/app_mode/kiosk_app_manager_browsertest.cc',
         'browser/chromeos/app_mode/kiosk_app_update_service_browsertest.cc',
         'browser/chromeos/attestation/attestation_policy_browsertest.cc',
         'browser/chromeos/drive/drive_integration_service_browsertest.cc',
         'browser/chromeos/drive/test_util.cc',
         'browser/chromeos/drive/test_util.h',
+        'browser/chromeos/extensions/screenlock_private_apitest.cc',
         'browser/chromeos/extensions/echo_private_apitest.cc',
         'browser/chromeos/extensions/file_manager/file_browser_handler_api_test.cc',
         'browser/chromeos/extensions/file_manager/file_browser_private_apitest.cc',
         'browser/chromeos/extensions/file_system_provider/file_system_provider_apitest.cc',
         'browser/chromeos/extensions/info_private_apitest.cc',
         'browser/chromeos/extensions/input_method_apitest_chromeos.cc',
         'browser/chromeos/extensions/virtual_keyboard_browsertest.cc',
         'browser/chromeos/extensions/wallpaper_private_apitest.cc',
         'browser/chromeos/file_manager/desktop_notifications_browsertest.cc',
         'browser/chromeos/file_manager/drive_test_util.cc',
@@ skipping 2217 matching lines @@
             'utility',
           ],
           'sources': [
             'tools/service_discovery_sniffer/service_discovery_sniffer.h',
             'tools/service_discovery_sniffer/service_discovery_sniffer.cc',
           ],
         }]
     }],
   ],  # 'conditions'
 }