[NaCl SDK] nacl_io: Auto-bind UDP socket on first sendto().

BUG=318983
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/63573006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235833 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235833