Fix linear-gradient definition in dataGrid.css

This was spotted by Eugene Klyuchnikov <eustas@chromium.org>
in https://codereview.chromium.org/152493003/.

Review URL: https://codereview.chromium.org/152033007

git-svn-id: svn://svn.chromium.org/blink/trunk@166535 bbb929c8-8fbe-4397-9dbb-9b2b20218538