[DevTools] Settings: fix column width on Mac for overrides tab.

In https://chromiumcodereview.appspot.com/18210005 dom structure has
been changed, but css selector wasn't updated.

BUG=271433

Review URL: https://chromiumcodereview.appspot.com/23039002

git-svn-id: svn://svn.chromium.org/blink/trunk@156093 bbb929c8-8fbe-4397-9dbb-9b2b20218538