[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,

[yhirano, 2014/10/08 09:31:39]

Maybe this will be used for the request mode transfer and can be removed from
this CL?

[horo, 2014/10/08 10:02:47]

This file is used from fetch-request-image.html.
This test is for the FetchEvent's request mode form the image tag which is set
in FetchRequest::setCrossOriginAccessControl()

+    };
+    event.respondWith(Promise.reject());
+  });