Update socket_libevent.cc to build on android

Android ships with crazy old system headers that don't include the
SOCK_NONBLOCK define. Since this is easy to live without, update the
domain socket code to set the socket to nonblocking using an fcntl when
built without this define. Still use SOCK_NONBLOCK when available since
it's cheaper to avoid the fcntl.

R=cmasone@chromium.org

Review URL: https://codereview.chromium.org/670773004