Move CSSStickyPosition back to status=test

We're not actively working on CSSStickyPosition. It's not ready for web
developers to experiment with yet.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/320323002

git-svn-id: svn://svn.chromium.org/blink/trunk@175816 bbb929c8-8fbe-4397-9dbb-9b2b20218538