CSS Parser: Treat var() as function instead of string

This patch changes the CSS grammar to parse var() references as
functions instead of string values. This is in preparation to add
an optional fallback value parameter to var().
This patch does not change existing behaviour.

BUG=330127

Review URL: https://codereview.chromium.org/108953008

git-svn-id: svn://svn.chromium.org/blink/trunk@165265 bbb929c8-8fbe-4397-9dbb-9b2b20218538