Tighten the let-not-count-the-UA-style-sheet logic

This is based on Eric's great comment on
https://codereview.chromium.org/23658040/.

Review URL: https://chromiumcodereview.appspot.com/23952007

git-svn-id: svn://svn.chromium.org/blink/trunk@158212 bbb929c8-8fbe-4397-9dbb-9b2b20218538