Block modal dialogs inside sandboxes.

LayoutTests/http/tests/security/popup-allowed-by-sandbox-is-sandboxed-control.html

 <script>
 if (window.testRunner) {
     testRunner.dumpAsText();
     testRunner.waitUntilDone();
     testRunner.setCanOpenWindows(true);
     testRunner.setCloseRemainingWindowsWhenComplete(true);
 }
 </script>
 <p>To run this test outside of DumpRenderTree, please disable your popup blocker!</p>
 <p>If you change this test, please be sure to change popup-allowed-by-sandbox-is-sandboxed.html as well!</p>
 <iframe sandbox="allow-scripts allow-popups allow-forms"
   src="data:text/html,
        <script>
-       var win = window.open('data:text/html,<form action=javascript:alert(/PASS/) ><input type=submit></form><script>document.forms[0].submit(); if (window.testRunner) testRunner.notifyDone();<\/script>', '_blank');
+       var win = window.open('data:text/html,<form action=javascript:console.log(/PASS/) ><input type=submit></form><script>document.forms[0].submit(); if (window.testRunner) testRunner.notifyDone();<\/script>', '_blank');
        </script>"
   ></iframe>