Notifying screen reader when a user changes settings on chrome://flags

When the banner "Your changes will take effect the next time you relaunch Chrome"
appears on the page, the screen reader will be alerted.

Bug: 982886
Change-Id: Ib148b8a3bc2c2949771ca5e949b9e42b2b182d19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696355Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Andrii Sagaidak <ansagaid@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#677033}

components/flags_ui/resources/flags.js

@@ -156,6 +156,9 @@ function resetAllFlags() {
  */
 function showRestartToast(show) {
   $('needs-restart').classList.toggle('show', show);
+  if (show) {
+    $('needs-restart').setAttribute("role", "alert");
+  }
 }
 
 /**