Temporarily mark ServiceWorker's unregister.html as pass/fail

Pending Chromium-side change crrev.com/247773004 alters the handling
of a "not found" unregistration from reject to resolve to match the
spec. Ignore test failures so that can land, then we'll update the test.

BUG=365926
NOTRY=true
TBR=michaeln@chromium.org

Review URL: https://codereview.chromium.org/246383005

git-svn-id: svn://svn.chromium.org/blink/trunk@172294 bbb929c8-8fbe-4397-9dbb-9b2b20218538