DevTools: Tune datagrid headers

Headers on heap snapshot containment view used to have double border.
That's fixed.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/131333006

git-svn-id: svn://svn.chromium.org/blink/trunk@166311 bbb929c8-8fbe-4397-9dbb-9b2b20218538