[Sheriff-o-matic] Use before content hax to hide category labels instead of ever-present spans

This isn't an ideal solution, but it hides the failure category labels when there are
no failure cards. Also adding a bit of styling in the meantime.

NOTRY=true
TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/555493005

git-svn-id: svn://svn.chromium.org/blink/trunk@181597 bbb929c8-8fbe-4397-9dbb-9b2b20218538