[ServiceWorker] Serialize request.referrer to "about:client" when it's client.

The spec of Fetch has been changed.
https://github.com/whatwg/fetch/commit/30c35ec10994d0005f45879778424e8fc7272bde

This change also renames "none" in "no referrer".
https://github.com/whatwg/fetch/commit/0576c5aaf0c21545001beb73aae9d8b585c6ece5

BUG=N/A
TEST=http/tests/serviceworker

Review URL: https://codereview.chromium.org/721103003

git-svn-id: svn://svn.chromium.org/blink/trunk@185282 bbb929c8-8fbe-4397-9dbb-9b2b20218538