[ServiceWorker] Set Fetch event request credentials mode correctly for resource fetching. 

BUG=418509
TEST=http/tests/serviceworker/

Review URL: https://codereview.chromium.org/687493002

git-svn-id: svn://svn.chromium.org/blink/trunk@184589 bbb929c8-8fbe-4397-9dbb-9b2b20218538