DevTools: Remove orphaned CSS rules [6]

See https://codereview.chromium.org/330223004

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/434613002

git-svn-id: svn://svn.chromium.org/blink/trunk@179377 bbb929c8-8fbe-4397-9dbb-9b2b20218538