Make the buttons on task-page work

appengine/swarming/elements/res/imp/common/auth-signin.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 
   The `auth-signin` element displays sign-in/sign-out button, user email and
   avatar.
   It has a google-signin/google-signin-aware element under the hood that handles
   the actual OAuth logic.
 
@@ skipping 66 matching lines @@
         },
         auth_response: {
           type: Object,
           notify: true,
         },
         client_id: {
           type: String,
         },
         profile: {
           type: Object,
-          readOnly: true
+          readOnly: true,
+          notify: true,
         },
         signed_in: {
           type: Boolean,
           readOnly: true,
           value: false,
           notify: true,
         },
       },
 
       attached: function() {
@@ skipping 65 matching lines @@
       signIn: function() {
         this.$.aware.signIn();
       },
 
       signOut: function() {
         this.$.aware.signOut();
       }
     });
   </script>
 </dom-module>