DevTools: fix DataGrid column alignment.

center/right alignment was lost in haste between revert and reland of
https://codereview.chromium.org/288973004/

BUG=372246

Review URL: https://codereview.chromium.org/293623002

git-svn-id: svn://svn.chromium.org/blink/trunk@174350 bbb929c8-8fbe-4397-9dbb-9b2b20218538