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

chrome/common/extensions/api/_api_features.json

Index: chrome/common/extensions/api/_api_features.json
diff --git a/chrome/common/extensions/api/_api_features.json b/chrome/common/extensions/api/_api_features.json
index e3ed97692ffb8d60ce3b0fb420ee4d51e5f7911b..f113072cfcbafa24c79c841af5b66ddc26206a3b 100644
--- a/chrome/common/extensions/api/_api_features.json
+++ b/chrome/common/extensions/api/_api_features.json
@@ -310,6 +310,12 @@
     "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"],