Add a11y description to ARC TOS screen

Bug: 1024092
Change-Id: If4adc02e604805b8cf9f312d0c7c931fa0126fa2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404934Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806617}

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

@@ -14,6 +14,8 @@
     <!-- As this dialog have pre-loading logic that require access to elements,
          dialog is marked as no-lazy. -->
     <oobe-dialog id="arcTosDialog" has-buttons class="arc-tos-loading"
+        role="dialog"
+        aria-label$="[[i18nDynamic(locale, 'arcTermsOfServiceScreenHeading')]]"
         title-key="arcTermsOfServiceScreenHeading"
         subtitle-key="arcTermsOfServiceScreenDescription" no-lazy>
       <iron-icon src="chrome://oobe/playstore.svg" slot="oobe-icon">