ServiceWorker: Deflake postmessage.html

postmessage.html sometimes fails due to an assertion failure.

Before this patch, the test expected to receive 'statechange' event for
'redundant' shortly after unregister() but it could be for 'activated'.
After this patch, the test correctly waits for 'redundant'.


BUG=n/a
TEST=run_webkit_tests.py --debug http/tests/serviceworker/postmessage.html

Review URL: https://codereview.chromium.org/584513002

git-svn-id: svn://svn.chromium.org/blink/trunk@182299 bbb929c8-8fbe-4397-9dbb-9b2b20218538