Deflake serviceworkerglobalscope-scope.html

The timeouts are likely caused by running two async tests with the
same scope concurrently. One thing that can happen is the first register
resolves and its worker reaches the active state, then the second
register resolves to the same value and wait_for_update will never return.

This patch makes the async tests run in sequence.

Review URL: https://codereview.chromium.org/488403002

git-svn-id: svn://svn.chromium.org/blink/trunk@180710 bbb929c8-8fbe-4397-9dbb-9b2b20218538