Measure the usage of user generated custom CSS pseudo element.

We already have the code to measure the usage of "pseudo" IDL attribute, but
it seemed be a good idea to also count the CSS usage.

BUG=242153
R=dglazkov@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22541003

git-svn-id: svn://svn.chromium.org/blink/trunk@155928 bbb929c8-8fbe-4397-9dbb-9b2b20218538