Service Worker: Don't let tests register in the default scope

Tests should use unique scopes so they don't interfere with each other.
The default scope is particularly dangerous because it means
layout tests themselves are controlled by the worker, if the test failed
to unregister.

This patch fixes a test that used the default scope and changes our
convenience function to not allow default scope.

Review URL: https://codereview.chromium.org/487773002

git-svn-id: svn://svn.chromium.org/blink/trunk@180620 bbb929c8-8fbe-4397-9dbb-9b2b20218538