Chrome OS A11Y - Fix privacy policy navigation

When clicking on "Google privacy policy" on the "Google Play apps
and services" screen the screen reader will treat the pop up as a
modal dialog and disable the navigation on the content behind it.

Tested locally.

Bug: 1024098
Change-Id: I924232eb7e423d579620146be4270e5ef43afc5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1950516Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Renato Silva <rrsilva@google.com>
Cr-Commit-Position: refs/heads/master@{#721916}

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

@@ -6,10 +6,11 @@
     role="group" hidden>
   <arc-tos-root id="arc-tos-root"></arc-tos-root>
 
-  <div id="arc-tos-overlay" class="popup-overlay" hidden>
+  <div id="arc-tos-overlay" class="popup-overlay"
+      role="dialog" aria-modal="true" hidden>
     <a href="#" id="arc-tos-overlay-start" class="arc-tos-invisible-link"></a>
     <div id="arc-tos-overlay-container" class="oobe-popup not-resizable flex
-        layout vertical stretch">
+        layout vertical stretch" role="dialog" aria-modal="true">
       <div id="arc-tos-overlay-close-top"
           class="arc-tos-overlay-close-top arc-overlay-close-button self-end">
       </div>