Allow login screen extensions to use background.persistent

This CL allows extensions of type login_screen_extension to use the
"persistent" field in the "background" manifest field.

Bug: 957573
Change-Id: If61a2b826ba2a818cc0f91ebff56c74c439b91e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631287
Auto-Submit: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664197}

extensions/common/api/_manifest_features.json

@@ -51,7 +51,7 @@
   "background.persistent": {
     "channel": "stable",
     "extension_types": [
-      "extension", "legacy_packaged_app"
+      "extension", "legacy_packaged_app", "login_screen_extension"
     ],
     "min_manifest_version": 2
   },