Sandbox the worker process on Mac

      When we transitioned to content it looks like we lost the Mac sandbox for workers, this CL re-enables it.

Also, update the comment in sandbox_init_mac.h

BUG=163208
TEST=Chrome should continue to work correctly.

Review URL: https://chromiumcodereview.appspot.com/11590006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@173432 0039d316-1c4b-4281-b951-d872f2087c98