Clean up some CSS nits in sheriff-o-matic

I've removed the horizontal line, which was just visual noise. I also moved the
margin and padding for the failure cards to their embedder, which is a better
practice.

NOTRY=true

Review URL: https://codereview.chromium.org/394323003

git-svn-id: svn://svn.chromium.org/blink/trunk@178295 bbb929c8-8fbe-4397-9dbb-9b2b20218538