gn win: Pass gperf path in css_properties generator

R=jamesr@chromium.org
BUG=354261

Review URL: https://codereview.chromium.org/473043002

git-svn-id: svn://svn.chromium.org/blink/trunk@180307 bbb929c8-8fbe-4397-9dbb-9b2b20218538