DevTools: Remove orphaned CSS rules [1]

See https://codereview.chromium.org/401013002

BUG=

Review URL: https://codereview.chromium.org/433893002

git-svn-id: svn://svn.chromium.org/blink/trunk@179316 bbb929c8-8fbe-4397-9dbb-9b2b20218538