(Re)organize handling of CORS access control during resource loading.

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

 <?php
 if (strtolower($_GET["cors"]) != "false") {
     header("Access-Control-Allow-Origin: http://127.0.0.1:8000");
 }
 if (strtolower($_GET["credentials"]) == "true") {
     header("Access-Control-ALlow-Credentials: true");
 }
 header("Content-Type: application/javascript");
 $value = $_GET['value'];
+if ($_SERVER['HTTP_ORIGIN'] && $_GET['value_cors']) {
+    $value = $_GET['value_cors'];
+}
 if ($value)
     echo "result = \"" . $value . "\";";
 else if (strtolower($_GET["fail"]) == "true")
     echo "throw({toString: function(){ return 'SomeError' }});";
 else
     echo "alert('script ran.');";
 ?>