Add missing runtime flag check for CSS variables in CSS parser

Without this check invalid expressions involving var() were being let
through while CSS variables were disabled.

BUG=339733

Review URL: https://codereview.chromium.org/142553004

git-svn-id: svn://svn.chromium.org/blink/trunk@166365 bbb929c8-8fbe-4397-9dbb-9b2b20218538