Block modal dialogs inside sandboxes.

LayoutTests/http/tests/security/no-indexeddb-from-sandbox.html

 <script>
 if (window.testRunner)
     testRunner.dumpAsText();
 </script>
 <iframe sandbox="allow-scripts"
     src="data:text/html,
          <script>
              try {
                  indexedDB.open('test');
-                 alert('FAIL: indexedDB.open() should throw a SECURITY_ERR in a sandbox.');
+                 console.log('FAIL: indexedDB.open() should throw a SECURITY_ERR in a sandbox.');
              } catch (e) {
                  if (e.code === DOMException.SECURITY_ERR)
-                     alert('PASS: indexedDB.open() threw a SECURITY_ERR!');
+                     console.log('PASS: indexedDB.open() threw a SECURITY_ERR!');
                  else
-                     alert('FAIL: indexedDB.open() threw a ' + e.name);
+                     console.log('FAIL: indexedDB.open() threw a ' + e.name);
              }
          </script>"
      ></iframe>