[ServiceWorker] Stop using FetchRequestData::Mode and FetchRequestData::Credentials.

They are duplication of WebURLRequest::FetchRequestMode and WebURLRequest::FetchCredentialsMode.

BUG=N/A

Review URL: https://codereview.chromium.org/706543002

git-svn-id: svn://svn.chromium.org/blink/trunk@184873 bbb929c8-8fbe-4397-9dbb-9b2b20218538