Add failing expectation for unregister-then-register-new-script.html

http/tests/serviceworker/unregister-then-register-new-script.html will
need to be rebaselined after the Chromium patch:
https://codereview.chromium.org/506043002/

BUG=398355
TBR=michaeln
NOTRY=true

Review URL: https://codereview.chromium.org/514523002

git-svn-id: svn://svn.chromium.org/blink/trunk@180931 bbb929c8-8fbe-4397-9dbb-9b2b20218538