[ServiceWorker] Set FetchRequestMode and handle wasFetchedViaServiceWorker.

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

+var requests = [];
+
+self.onmessage = function(e) {
+  var message = e.data;
+  if ('port' in message) {
+    var port = message.port;
+    port.postMessage(requests);
+  }
+};
+
+self.addEventListener('fetch', function(event) {
+    var url = event.request.url;
+    if (url.indexOf('dummy?test') == -1) {
+      return;
+    }
+    requests[url] = {
+      mode: event.request.mode,
+    };
+    event.respondWith(Promise.reject());
+  });