[ServiceWorker] Remove the test for the rejected result of FetchEvent from fetch-event.html

We don't have the way to observe the network error using <iframe>.
So this change removes the rejecting test from fetch-event.html

I will introduce the reject test using XMLHttpRequest later. (step 3)

Step1: this patch
  [ServiceWorker] Remove the test for the rejected result of FetchEvent from fetch-event.html
  https://codereview.chromium.org/588993002/
Step2: [ServiceWorker] Treat the rejected result of onFetch as a Network Error.
  https://codereview.chromium.org/590043002/
Step3: [ServiceWorker] Add test for the rejected result of FetchEvent using XHR.
 https://codereview.chromium.org/588093003/

BUG=411173

Review URL: https://codereview.chromium.org/588993002

git-svn-id: svn://svn.chromium.org/blink/trunk@182401 bbb929c8-8fbe-4397-9dbb-9b2b20218538