[ServivceWorker] Treat rejecting respondWith as a Network Error

Chromium side change is this: https://codereview.chromium.org/574643003/


BUG=411173
TEST=layouttest http/tests/serviceworker/

Review URL: https://codereview.chromium.org/571843003

git-svn-id: svn://svn.chromium.org/blink/trunk@182147 bbb929c8-8fbe-4397-9dbb-9b2b20218538