Remove CSS Variables from experimental features

CSS Variables currently have some performance issues to resolve.
This patch will disable CSS Variables during performance testing
while these issues are being worked on.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/145993003

git-svn-id: svn://svn.chromium.org/blink/trunk@165675 bbb929c8-8fbe-4397-9dbb-9b2b20218538