document.lastModified: treat invalid dates like unknown ones.

third_party/WebKit/LayoutTests/http/tests/misc/last-modified-parsing.html

 <script>
 if (window.testRunner) {
     testRunner.dumpAsText();
     testRunner.dumpChildFramesAsText();
 }
 
 function getExpectedLastModified(lastModified) {
     var date = new Date(lastModified);
+    if (isNaN(date.getTime()))
+      return "illformed-date";
     var month = ('0' + (date.getMonth() + 1)).slice(-2);
     var day = ('0' + date.getDate()).slice(-2);
     var hour = ('0' + date.getHours()).slice(-2);
     var minute = ('0' + date.getMinutes()).slice(-2);
     var second = ('0' + date.getSeconds()).slice(-2);
     var result = month + '/' + day + '/' + date.getFullYear() + ' ' + hour + ':' + minute + ':' + second;
     return result;
 }
 
 function test(text) {
     var url = '../resources/last-modified.php?date=' + escape(text) + '&expected=' + escape(getExpectedLastModified(text));
     document.write('<iframe id="test" src="' + url + '"></iframe>');
 }
 
 test('Mon, 21 Nov 2008 01:03:33 GMT');
 test('Tuesday, 21 Nov 2008 01:03:33 GMT');
 test('Mon, 21 Nov 97 01:03:33 GMT');
 test('Mon, 21-Nov-2008 01:03:33 GMT');
 test('Mon, 21-Feb-2008 01:03:33 GMT');
 test('Mon, 03-Feb-2008 01:03:33 GMT');
 test('Mon, 3-Mar-2008 01:03:33 GMT');
+test('Something ill-formed');
 </script>