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

chrome/test/data/extensions/api_test/screenlockPrivate/test.js

+// Copyright (c) 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.
+
+var stage = 0;
+
+function step() {
+  chrome.screenlockPrivate.getLocked(function gotLocked(locked) {
+    if (stage === 0) {
+      if (locked) {
+        chrome.test.fail('locked at stage ' + stage);
+      } else {
+        chrome.screenlockPrivate.setLocked(true);
+        ++stage;
+        return true;
+      }
+    } else if (stage === 1) {
+      if (!locked) {
+        chrome.test.fail('unlocked at stage ' + stage);
+      } else {
+        chrome.screenlockPrivate.setLocked(false);
+        ++stage;
+        return true;
+      }
+    } else if (stage === 2) {
+      if (locked) {
+        chrome.test.fail('locked at stage ' + stage);
+      } else {
+        chrome.test.succeed();
+      }
+    }
+  });
+};
+
+chrome.screenlockPrivate.onChanged.addListener(function(locked) {
+  if (locked != (1 == (stage % 2))) {
+    chrome.test.fail((locked ? '' : 'un') + 'locked at stage ' + stage +
+                     ' onChanged');
+  } else {
+    return step();
+  }
+});
+
+step();