CrOS enrollment: announce enrollment in progress

This informs the user that enrollment is on-going. The 'alert' role is
probably not perfect for this use case but it's the best I could find,
and it's used in nearby code for similar scenarios.

Testing by running through the remora enrollment flow and observing that
"Enrolling device..." is read out.

BUG=b:80126209

Change-Id: Iee7388dae29458412cb44f9d3578cfeee2d234cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559182
Auto-Submit: Felix Ekblom <felixe@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649228}

chrome/browser/resources/chromeos/login/oobe_screen_oauth_enrollment.html

@@ -32,7 +32,7 @@
           <paper-progress slot="progress" indeterminate>
           </paper-progress>
 
-          <div slot="footer" class="flex layout vertical">
+          <div slot="footer" class="flex layout vertical" role="alert">
             <div class="oauth-enroll-step-message">
               <span i18n-content="oauthEnrollWorking"></span>
             </div>