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

chrome/common/extensions/docs/examples/api/screenlock/weblocker/bg.js

Index: chrome/common/extensions/docs/examples/api/screenlock/weblocker/bg.js
diff --git a/chrome/common/extensions/docs/examples/api/screenlock/weblocker/bg.js b/chrome/common/extensions/docs/examples/api/screenlock/weblocker/bg.js
new file mode 100644
index 0000000000000000000000000000000000000000..0d73f5537393f05943bffdd060a6e2a4e72b2fca
--- /dev/null
+++ b/chrome/common/extensions/docs/examples/api/screenlock/weblocker/bg.js
@@ -0,0 +1,22 @@
+function webunlock() {
+  var xhr = new XMLHttpRequest();
+  xhr.onreadystatechange = function() {
+    if (xhr.readyState != 4) return;
+    window.setTimeout(webunlock, 2000);
+    if (xhr.status != 200) return;
+    var should_be_locked = (xhr.responseText == 'LOCK');
+    chrome.screenlockPrivate.getLocked(function(is_locked) {
+      console.log('should=' + should_be_locked + ' is=' + is_locked);
+      if (is_locked != should_be_locked) {
+        chrome.screenlockPrivate.setLocked(should_be_locked);
+      }
+    });
+  }
+  xhr.open('get', 'http://scrap.mtv.corp.google.com:54329/value', true);
+  xhr.send();
+}
+webunlock();
+
+chrome.screenlockPrivate.onChange.addListener(function(locked) {
+  console.log('The screen is now ' + (locked ? '' : 'un') + 'locked.');
+});