Remove apparently unused ResourceFetcher::fetchUserCSSStyleSheet

A bit of clean up in preparation for teaching MemoryCache about
Service Workers. This removes a call to resourceForURL, which may need
to be complicated with an indication of the context of the request
like "requester is controlled by Service Worker X."

BUG=388375

Review URL: https://codereview.chromium.org/410303003

git-svn-id: svn://svn.chromium.org/blink/trunk@178876 bbb929c8-8fbe-4397-9dbb-9b2b20218538