Rework EventSource CORS tests to be usable from Workers.

LayoutTests/http/tests/eventsource/script-tests/eventsource-cors-basic.js

+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test that basic EventSource cross-origin requests fail until they are allowed by the Access-Control-Allow-Origin header.");
+
+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-basic.php?count=" + count + "\")");
+    shouldBeFalse("es.withCredentials");
+    es.onerror = function () {
+        if (es.readyState == es.CLOSED) {
+            shouldBe("es.readyState", "es.CLOSED");
+            shouldBeTrue("count != 3 && count != 4");
+            count++;
+            setTimeout(runTest);
+            return;
+        }
+        shouldBe("count", "4");
+    };
+    es.onmessage = function (arg) {
+        evt = arg;
+        shouldBeTrue("evt.origin.indexOf('http://127.0.0.1:8080') === 0");
+        if (count == 3) {
+            shouldBeTrue("evt.data == 'DATA1' && evt.lastEventId == '77'");
+            count++;
+            return;
+        }
+        shouldBe("count", "4");
+        shouldBeEqualToString("evt.data", "DATA2");
+        debug("DONE");
+        es.close();
+        finishJSTest();
+    };
+}
+runTest();