[Extensions] Add test case for extensions on sign-in screen

This CL...
* adds a new extension (also hosted on chrome webstore), which is
  whitelisted for the sign-in screen.
* adds a new test case for this whitelisted extension to
  signin_profile_apps_policy_browsertest.cc
* checks that the whitelisted extension has the new
  "login_screen_extension" type introduced in CL 1533910.


DD: go/dd-login-screen-extensions

Bug: 957573, 938943
Test: unit_tests --gtest_filter=SigninProfileExtensionsPolicy*
Change-Id: I184235439d7aa0534cc1b72f90e144e5b12e11c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1584275
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661707}

chrome/test/BUILD.gn

@@ -2003,7 +2003,7 @@ test("browser_tests") {
         "../browser/chromeos/policy/login_screen_default_policy_browsertest.cc",
         "../browser/chromeos/policy/power_policy_browsertest.cc",
         "../browser/chromeos/policy/restore_on_startup_browsertest_chromeos.cc",
-        "../browser/chromeos/policy/signin_profile_apps_policy_browsertest.cc",
+        "../browser/chromeos/policy/signin_profile_extensions_policy_browsertest.cc",
         "../browser/chromeos/policy/site_isolation_flag_handling_browsertest.cc",
         "../browser/chromeos/policy/status_collector/child_status_collector_browsertest.cc",
         "../browser/chromeos/policy/status_collector/device_status_collector_browsertest.cc",

chrome/test/data/extensions/signin_screen_manual_test_extension/OWNERS

+hendrich@chromium.org

chrome/test/data/extensions/signin_screen_manual_test_extension/README

+The extension_signed_by_webstore.crx package must be a one signed by WebStore,
+in order for the extension to have the expected ID which is whitelisted in
+Chrome - "ngjobkbdodapjbbncmagbccommkggmnj".
+
+This extension is primarily intended to be used for the manual testing of the
+extensions on the Chrome OS sign-in screen.
+
+In case a new version of the test extension has to be published on WebStore,
+please reach out to the managed-devices@ mailing list.

chrome/test/data/extensions/signin_screen_manual_test_extension/extension/background.js

+// Copyright 2017 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.
+
+console.log('The Trivial Test Extension background page loaded');

chrome/test/data/extensions/signin_screen_manual_test_extension/extension/manifest.json

+{
+  "name": "Sign-in Screen Test Extension",
+  "version": "2.0",
+  "manifest_version": 2,
+  "description": "The extension for manual testing of the extensions installation in the Chrome OS sign-in profile",
+  "background": {
+    "scripts": ["background.js"]
+  }
+}

chrome/test/data/extensions/signin_screen_manual_test_extension/update_manifest.xml

+<?xml version='1.0' encoding='UTF-8'?>
+<!--
+  This update manifest points to the ./extension.crx file. "mock.http" is a
+  placeholder that gets substituted with the test server address in runtime.
+-->
+<gupdate xmlns='http://www.google.com/update2/response' protocol='2.0'>
+  <app appid='ngjobkbdodapjbbncmagbccommkggmnj'>
+    <updatecheck
+        codebase='http://mock.http/extensions/signin_screen_manual_test_extension/extension_signed_by_webstore.crx'
+        version='2.0' />
+   </app>
+</gupdate>