Make window.navigator property non replaceable

LayoutTests/fast/js/var-declarations-shadowing.html

 <p>
 This page tests var declarations that attempt to shadow interesting global properties in
 interesting ways. <a href="http://bugs.webkit.org/show_bug.cgi?id=16644">Bug 16644</a>.
 </p>
 
 <p>
 If the test passes, you'll see a series of PASS messages below.
 </p>
 
 <pre id="console"></pre>
@@ skipping 202 matching lines @@
 
 try {
     eval("var devicePixelRatio = marker");
 } catch(e) { }
 shouldBe(devicePixelRatio == marker, "devicePixelRatio == marker", true);
 shouldBe(eval('devicePixelRatio == marker'), "eval('devicePixelRatio == marker')", true);
 
 try {
     eval("var navigator = marker");
 } catch(e) { }
-shouldBe(navigator == marker, "navigator == marker", true);
-shouldBe(eval('navigator == marker'), "eval('navigator == marker')", true);
+shouldBe(navigator == marker, "navigator == marker", false);
+shouldBe(eval('navigator == marker'), "eval('navigator == marker')", false);
 
 try {
     eval("var clientInformation = marker");
 } catch(e) { }
 shouldBe(clientInformation == marker, "clientInformation == marker", true);
 shouldBe(eval('clientInformation == marker'), "eval('clientInformation == marker')", true);
 
 try {
     eval("var status = marker");
 } catch(e) { }
@@ skipping 68 matching lines @@
 shouldBe(crypto == marker, "crypto == marker", false);
 shouldBe(eval('crypto == marker'), "eval('crypto == marker')", false);
 
 try {
     eval("var top = marker");
 } catch(e) { }
 shouldBe(top == marker, "top == marker", false);
 shouldBe(eval('top == marker'), "eval('top == marker')", false);
 
 </script>