document.lastModified: treat invalid dates like unknown ones.

third_party/WebKit/LayoutTests/http/tests/resources/last-modified.php

 <?php
     $date = $_GET['date'];
     $expected = $_GET['expected'];
     header("Last-Modified: $date");
 ?>
 <script>
 var expected = '<?=$expected?>';
 var lastModified = document.lastModified;
-if (expected == lastModified) {
+
+// If expected is 'illformed-date', then the Last-Modified: header
+// is assumed to contain an unparseable date. For which we're supposed
+// to return the current time per spec. Simply verify that by checking
+// that the |lastModified| parses.
+if (expected === lastModified ||
+    (expected === "illformed-date" && !isNaN(Date.parse(lastModified)))) {
     document.write('PASS');
 } else {
     document.write('FAIL: expect ' + expected + ', but got ' + lastModified);
 }
 </script>