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

chrome/common/extensions/api/_api_features.json

 // Copyright (c) 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.
 
 // See c/c/e/features/* to understand this file, in particular feature.h,
 // simple_feature.h, and base_feature_provider.h.
 
 {
   "activityLogPrivate": {
     "dependencies": ["permission:activityLogPrivate"],
@@ skipping 292 matching lines @@
     "platforms": ["chromeos"],
     "dependencies": ["permission:fileBrowserHandler"],
     "contexts": ["blessed_extension"]
   },
   "fileBrowserHandlerInternal": {
     "internal": true,
     "platforms": ["chromeos"],
     "channel": "stable",
     "contexts": ["blessed_extension", "unblessed_extension", "content_script"]
   },
+  "screenlockPrivate": {
+    "platforms": ["chromeos"],
+    "dependencies": ["permission:screenlockPrivate"],
+    "extension_types": ["platform_app"],
+    "contexts": ["blessed_extension", "unblessed_extension"]
+  },
   "fileBrowserPrivate": {
     "platforms": ["chromeos"],
     "dependencies": ["permission:fileBrowserPrivate"],
     "contexts": ["blessed_extension"]
   },
   "fileSystem": {
     "dependencies": ["permission:fileSystem"],
     "contexts": ["blessed_extension"]
   },
   "fileSystemProvider": {
@@ skipping 403 matching lines @@
     "internal": true,
     "channel": "stable",
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }