Avoid counting properties from the UA style sheet

The current logic count properties from the UA style sheet, which is
introducing a bias in our measurement as we are only interested in
the author's style sheets and not whatever we inject.

We belive this bias is small as we cache the parsed UA style sheet
(the caching logic is quite complicated so it's difficult to give
some real number) but it still taints our numbers.

Review URL: https://chromiumcodereview.appspot.com/23658040

git-svn-id: svn://svn.chromium.org/blink/trunk@157727 bbb929c8-8fbe-4397-9dbb-9b2b20218538