Allow sandboxed OS X processes to open /dev/urandom

Match Linux/POSIX and Windows in allowing sandboxed processes access to
OS entropy (RtlGenRandom on Win, /dev/urandom on Linux/POSIX).

BUG=242702
R=jeremy
TBR=cevans@chromium.org, piman@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15637004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202267 0039d316-1c4b-4281-b951-d872f2087c98