Make throbber_notice accessible

Bug: 1021887
Change-Id: I09c1792a4e9d51c2336e0d00dad6350a41591409
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1959037Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723442}

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

@@ -11,6 +11,8 @@
   <link rel="stylesheet" href="throbber_notice.css">
   <template>
     <paper-spinner-lite dir="ltr" active></paper-spinner-lite>
-    <div id="spinner-comment"><span>[[text]]</span></div>
+    <div id="spinner-comment" aria-live="polite" aria-label="[[text]]"
+      [[text]]
+    </div>
   </template>
 </dom-module>