Misc style fixes and cleanup in ServiceWorker.

Remove a few style violations and redundancies.

R=falken@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/457953002

git-svn-id: svn://svn.chromium.org/blink/trunk@180593 bbb929c8-8fbe-4397-9dbb-9b2b20218538