[ServiceWorker] Add LayoutTests for the fallback behavior of FetchEvent.

LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js

+var requests = [];
+
+self.addEventListener('message', function(event) {
+    event.data.port.postMessage({requests: requests});
+  });
+
+self.addEventListener('fetch', function(event) {
+    requests.push({
+        url: event.request.url,
+        mode: event.request.mode
+      });
+  });