Throw exceptions on all failed cross-origin access checks.

LayoutTests/http/tests/security/xss-DENIED-window-index-assign.html

 <script>
 if (window.testRunner)
     testRunner.dumpAsText();
 
 window[0] = {
     a: "1",
     f: function() {
         alert("FAIL: Child called parent.f()");
     }
 };
 </script>
-<iframe src="data:text/html,<script>alert(parent[0].a);</script><script>parent[0].f();</script>"></iframe><br>
+<iframe src="data:text/html,<script>try { alert(parent[0].a); } catch (e) { alert('PASS: Access to parent[0].a threw exception.'); }</script><script>try { parent[0].f(); } catch (e) { alert('PASS: Access to parent[0].f() threw exception.'); }</script>"></iframe><br>
 This test passes if the access is forbidden.