Correctly handle CORS access checks over cache-validating responses.

LayoutTests/http/tests/security/resources/conditional-cors.php

Index: LayoutTests/http/tests/security/resources/conditional-cors.php
diff --git a/LayoutTests/http/tests/security/resources/conditional-cors.php b/LayoutTests/http/tests/security/resources/conditional-cors.php
new file mode 100644
index 0000000000000000000000000000000000000000..eb2080cbd04df02259510e002f0dc27b5485f949
--- /dev/null
+++ b/LayoutTests/http/tests/security/resources/conditional-cors.php
@@ -0,0 +1,10 @@
+<?php
+if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
+   header('HTTP/1.0 304 Not Modified');
+   exit;
+}
+header('Access-Control-Allow-Origin: *');
+header('Last-Modified: ' . gmdate("D, d M Y H:i:s", filemtime("conditional-cors.php")) . " GMT");
+header('Content-Type: application/javascript');
+echo '2;'
+?>