Rework EventSource tests for better Worker test coverage.

LayoutTests/http/tests/eventsource/script-tests/eventsource-reconnect.js

+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test EventSource reconnect after end of event stream.");
+
+self.jsTestIsAsync = true;
+
+var retryTimeout;
+
+function checkReadyState(es, desiredState) {
+    shouldBe("es.readyState", "es." + desiredState);
+}
+
+var errCount = 0;
+var es = new EventSource("/eventsource/resources/reconnect.php");
+
+checkReadyState(es, "CONNECTING");
+
+es.onopen = function (evt) {
+    checkReadyState(es, "OPEN");
+};
+
+var evt;
+es.onmessage = function (arg) {
+    evt = arg;
+    if (errCount)
+        shouldBeEqualToString("evt.data", "77");
+    shouldBeEqualToString("evt.lastEventId", "77");
+};
+
+es.onerror = function () {
+    errCount++;
+    if (errCount < 2) {
+        checkReadyState(es, "CONNECTING");
+        retryTimeout = setTimeout(end, 1000);
+        return;
+    }
+    clearTimeout(retryTimeout);
+    retryTimeout = null;
+    end();
+};
+
+function end() {
+    es.close();
+    if (retryTimeout)
+        testFailed("did not reconnect in time");
+    else
+        checkReadyState(es, "CLOSED");
+
+    finishJSTest();
+}