Make accessibility/aria-checkbox-sends-notification.html non-flaky

The test was trying to use the JS test infrastructure, but because it wasn't
using jsTestIsAsync and finishJSTest(), its output was not consistent.

R=dmazzoni@google.com

Review URL: https://chromiumcodereview.appspot.com/20384004

git-svn-id: svn://svn.chromium.org/blink/trunk@154952 bbb929c8-8fbe-4397-9dbb-9b2b20218538