Add alert role for demo toast with "RETRY" button

Author: dfreedm

demo.html

@@ -44,7 +44,7 @@
 
   <paper-toast id="toast1" text="Your draft has been discarded."></paper-toast>
 
-  <paper-toast id="toast2" text="Connection timed out. Showing limited messages.">
+  <paper-toast id="toast2" role="alert" text="Connection timed out. Showing limited messages.">
     <div style="color: #eeff41;" onclick="console.log('RETRY')">Retry</div>
   </paper-toast>