DevTools: Remove orphaned CSS rules [A]

See https://codereview.chromium.org/155823002

BUG=

Review URL: https://codereview.chromium.org/431053002

git-svn-id: svn://svn.chromium.org/blink/trunk@179319 bbb929c8-8fbe-4397-9dbb-9b2b20218538