ServiceWorker: IDL tweaks to match spec

Trivial IDL changes: replace a DOMString with an enum and mark some
attributes nullable. Since they're all readonly there's no behavior
change - this is just for spec alignment.

Review URL: https://codereview.chromium.org/374163002

git-svn-id: svn://svn.chromium.org/blink/trunk@177706 bbb929c8-8fbe-4397-9dbb-9b2b20218538