third_party/WebKit/ManualTests/back-forward-during-alert-2.html

<script>
function loaded()
{
   alert("While this alert is showing, two-fingered swipe back using a MacBook trackpad, Magic Trackpad, or Magic Mouse.  You should not be able to navigate back.  If you can, dismissing this dialog will likely result in a crash.");
}
</script>

<body onload="loaded();">
Follow the instructions in the alert that shows up on page load.
</body>