Add missing runtime flag check for CSS variables in CSS parser
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@166365 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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