Test of sign-in screen extensions auto-update

Add a browser test for the scenario when the sign-in screen extension
is first installed at some version, and then the server starts
announcing a newer version, so that Chrome eventually fetches, installs
and runs it.

Bug: 1012892
Change-Id: Iad3537236d56d7c86e02478bd2ff46fea6c0d112
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992420
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Oleg Davydov <burunduk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730201}

chrome/test/data/extensions/signin_screen_manual_test_app/README

-The app_signed_by_webstore.crx package must be a one signed by WebStore, in
-order for the app to have the expected ID which is whitelisted in Chrome -
-"bjaiihebfngildkcjkjckolinodhliff".
-
 This app is primarily intended to be used for the manual testing of the apps on
-the Chrome OS sign-in screen.
+the Chrome OS sign-in screen. The sources are located under the ./app/
+directory.
+
+The ./crx/**/app_signed_by_webstore.crx packages must be files signed by
+WebStore, in order for the app to have the expected ID which is whitelisted in
+Chrome - "bjaiihebfngildkcjkjckolinodhliff".
+
+Browser tests in Chrome currently require these packages for two different
+versions.
 
 In case a new version of the test app has to be published on WebStore, please
 reach out to the managed-devices@ mailing list.

chrome/test/data/extensions/signin_screen_manual_test_app/update_manifest.xml → chrome/test/data/extensions/signin_screen_manual_test_app/crx/3.0/update_manifest.xml

@@ -7,7 +7,7 @@
 <gupdate xmlns='http://www.google.com/update2/response' protocol='2.0'>
   <app appid='bjaiihebfngildkcjkjckolinodhliff'>
     <updatecheck
-        codebase='http://mock.http/extensions/signin_screen_manual_test_app/app_signed_by_webstore.crx'
+        codebase='http://mock.http/extensions/signin_screen_manual_test_app/crx/3.0/app_signed_by_webstore.crx'
         version='3.0' />
    </app>
 </gupdate>

chrome/test/data/extensions/signin_screen_manual_test_app/crx/4.0/update_manifest.xml

+<?xml version='1.0' encoding='UTF-8'?>
+<!--
+  This update manifest points to the ./app_signed_by_webstore.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='bjaiihebfngildkcjkjckolinodhliff'>
+    <updatecheck
+        codebase='http://mock.http/extensions/signin_screen_manual_test_app/crx/4.0/app_signed_by_webstore.crx'
+        version='4.0' />
+   </app>
+</gupdate>