ServiceWorker: Add layout tests for fetch event response

LayoutTests/http/tests/serviceworker/resources/fetch-event-test-worker.js

+function handleHelloWorld(event) {
+    event.respondWith(new Response(new Blob(["hello, world"])));
+}
+
+function handleNullBody(event) {
+    event.respondWith(new Response(null));
+}
+
+function handleReject(event) {
+    event.respondWith(new Promise(function(resolve, reject) {
+        reject('rejected!');
+    }));
+}
+
+self.addEventListener('fetch', function(event) {
+    var url = event.request.url;
+    var handlers = [
+        { pattern: 'helloworld', fn: handleHelloWorld },
+        { pattern: '?ignore', fn: function() {} },
+        { pattern: '?null', fn: handleNullBody },
+        { pattern: '?reject', fn: handleReject }
+    ];
+
+    var handler = null;
+    for (var i = 0; i < handlers.length; ++i) {
+       if (url.indexOf(handlers[i].pattern) != -1) {
+           handler = handlers[i];
+           break;
+       }
+    }
+
+    if (handler)
+        handler.fn(event);
+    else
+        event.respondWith(new Response(new Blob(['Service Worker got an unexpected request: ' + url])));
+});