Rework EventSource CORS tests to be usable from Workers.

LayoutTests/http/tests/eventsource/script-tests/eventsource-cors-with-credentials.js

+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test that EventSource cross-origin requests with credentials fail until the correct CORS headers are sent.");
+
+self.jsTestIsAsync = true;
+
+var count = 1;
+var es, evt;
+
+function runTest() {
+    shouldNotThrow("es = new EventSource(\"http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=" + count + "\", {'withCredentials': true})");
+    shouldBeTrue("es.withCredentials");
+    es.onerror = function () {
+        if (es.readyState == es.CLOSED) {
+            shouldBeTrue("count != 4 && count != 5");
+            count++;
+            setTimeout(runTest);
+            return;
+        }
+        shouldBe("count", "5");
+    };
+    es.onmessage = function (arg) {
+        evt = arg;
+        shouldBeTrue("evt.origin.indexOf('http://127.0.0.1:8080') === 0");
+        if (count == 4) {
+            shouldBeEqualToString("evt.data", "DATA1");
+            shouldBeEqualToString("evt.lastEventId", "77");
+            count++;
+            return;
+        }
+        shouldBe("count", "5");
+        shouldBeEqualToString("evt.data", "DATA2");
+        es.close();
+        finishJSTest();
+    };
+}
+runTest();