Use flock on OpenBSD.

This undoes the change made in https://codereview.chromium.org/12389082 .

I have tested this on actual OpenBSD 5.2 system, and this is also
what OpenBSD www/chromium port uses.

BUG=none
Review URL: https://codereview.chromium.org/12719010/

Patch from Paweł Hajdan Jr. <phajdan.jr@chromium.org>!

git-svn-id: http://gyp.googlecode.com/svn/trunk@1601 78cadc50-ecff-11dd-a971-7dbc132099af