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

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

+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);

[Matt Perry, 2013/11/13 01:01:14]

I wouldn't check this in with a reference to a corp URL.

[Matt Perry, 2013/11/13 01:01:14]

Do you need this example now that there's an API test?

[benjhayden, 2013/11/13 17:17:51]

Done.

[benjhayden, 2013/11/13 17:17:51]

Done.

+  xhr.send();
+}
+webunlock();
+
+chrome.screenlockPrivate.onChanged.addListener(function(locked) {
+  console.log('The screen is now ' + (locked ? '' : 'un') + 'locked.');
+});