Add CSP header for resources with an active policy

LayoutTests/http/tests/security/contentSecurityPolicy/resources/csp-header-is-sent.js

+async_test(function(t) {
+    var xhr = new XMLHttpRequest();
+    xhr.onload = function () {
+        t.step(function () { assert_true(xhr.status === 404); t.done(); });
+    };
+    // Send a request that returns 200 if and only if CSP header is present.
+    xhr.open(
+        "GET",
+        "http://127.0.0.1:8000/security/contentSecurityPolicy/resources/test-csp-header.pl",
+        true
+    );
+    xhr.send();
+}, 'CSP header is not sent on resource requests for which there is no policy.');
+
+test(function() {
+    assert_true(typeof(script_loaded) !== "undefined");
+}, 'CSP header is sent on resource requests for which there is a policy.');