DevTools: Remove orphaned CSS rules [3]

See https://bugs.webkit.org/show_bug.cgi?id=72920

BUG=

Review URL: https://codereview.chromium.org/438603002

git-svn-id: svn://svn.chromium.org/blink/trunk@179311 bbb929c8-8fbe-4397-9dbb-9b2b20218538