Null out LocalDOMWindow::m_frame on navigation.

third_party/WebKit/LayoutTests/http/tests/security/resources/xss-eval2.html

 <script>
 document.testExpando = "It's me too!";
 
 parent.childEval = eval;
 
-parent.childEvalCaller = function(s) {
-    return window.eval(s);
-}
+parent.childEvalCaller = (function()
+{
+    // Capture window.window into a variable, since this property always returns null once
+    // the context is navigated.
+    var w = window;
+    return function(s) {
+        return w.eval(s);
+    }
+})();
 
 parent.childLocalEvalCaller = (function()
 {
     var e = eval;
     return function(s) { return e(s); };
 })();
 
 location.href = "http://localhost:8000/security/resources/xss-eval3.html";
 </script>