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

BUG=418509
TEST=http/tests/serviceworker/

Review URL: https://codereview.chromium.org/682383004

git-svn-id: svn://svn.chromium.org/blink/trunk@184591 bbb929c8-8fbe-4397-9dbb-9b2b20218538