Block modal dialogs inside sandboxes.

LayoutTests/fast/frames/sandboxed-iframe-parsing-space-characters.html

 This tests whether we correct parse various space characters in the sandbox attribute.<br>
 <script>
 var testCases = [
     [' ', 'PASS: Space is a delimiter character.'],
     ['\t', 'PASS: Tab is a delimiter.'],
     ['x', 'FAIL: x is not a delimiter.'],
     ['\r', 'PASS: Return is a delimiter.'],
     ['\n', 'PASS: Newline is a delimiter.'],
     ['\v', 'FAIL: Vertical tab is not a delimiter.'],
     ['\f', 'PASS: Form feed is a delimiter.'],
 ]
 
 function next() {
     if (testCases.length) {
         var testCase = testCases.pop();
         testCharacter.apply(null, testCase);
         return;
     }
 
     if (window.testRunner)
         testRunner.notifyDone();
 }
 
 function testCharacter(possibleDelimiter, message) {
     var policy = "allow-scripts" + possibleDelimiter + "allow-forms";
     var iframe = document.createElement('iframe');
     iframe.sandbox = policy;
-    iframe.src = "data:text/html,<script>alert('" + message + "');<\/script>";
+    iframe.src = "data:text/html,<script>console.log('" + message + "');<\/script>";
     iframe.onload = next;
     document.body.appendChild(iframe);
 }
 
 if (window.testRunner) {
     testRunner.dumpAsText();
     testRunner.waitUntilDone();
 }
 
 next();
 
 </script>