[ServiceWorker] Handle ServiceWorker script fetch error code correctly. [blink]

Chromium side fix is here: https://codereview.chromium.org/660973002

BUG=424464

Review URL: https://codereview.chromium.org/663723002

git-svn-id: svn://svn.chromium.org/blink/trunk@183878 bbb929c8-8fbe-4397-9dbb-9b2b20218538