Add a definition for "is_bsd"

Needed for Chromium code reviews 138003017 and 146963007.

BUG=

Landing for r.c.ladan@gmail.com, patch from https://codereview.chromium.org/156493003

Review URL: https://codereview.chromium.org/158743002

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@151 7262f16d-afe8-6277-6482-052fa10e57b1