[ServiceWorker] Stop using FetchRequestData::Context

This is duplication of WebURLRequest::RequestContext.

BUG=N/A

Review URL: https://codereview.chromium.org/726503002

git-svn-id: svn://svn.chromium.org/blink/trunk@185278 bbb929c8-8fbe-4397-9dbb-9b2b20218538