ServiceWorker: unregister's scope argument should be optional

Bring implementation in line with the latest spec edits: the |scope|
argument to unregister() should be optional and default to '/*'.

R=dominicc@chromium.org,michaeln@chromium.org,falken@chromium.org
BUG=388044

Review URL: https://codereview.chromium.org/362123003

git-svn-id: svn://svn.chromium.org/blink/trunk@178034 bbb929c8-8fbe-4397-9dbb-9b2b20218538