ServiceWorker: unregister-then-register-new-script.html should be skipped

Currently this test is expected to be timed out and should be skipped to reduce
the time to run tests.

BUG=398355
TBR=horo@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/546333002

git-svn-id: svn://svn.chromium.org/blink/trunk@181535 bbb929c8-8fbe-4397-9dbb-9b2b20218538