Add code to set TCP keep alive delay on OSX.

net/socket/tcp_socket_libevent.cc

Index: net/socket/tcp_socket_libevent.cc
diff --git a/net/socket/tcp_socket_libevent.cc b/net/socket/tcp_socket_libevent.cc
index c5d30e1d5b085ec3781c3ca74044f7402fbd5283..187f5d34ae725010743fdb47eb14ef7ece4177d6 100644
--- a/net/socket/tcp_socket_libevent.cc
+++ b/net/socket/tcp_socket_libevent.cc
@@ -77,6 +77,11 @@ bool SetTCPKeepAlive(int fd, bool enable, int delay) {
     PLOG(ERROR) << "Failed to set TCP_KEEPINTVL on fd: " << fd;
     return false;
   }
+#elif defined(OS_MACOSX) || defined(OS_IOS)
+  if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPALIVE, &delay, sizeof(delay))) {
+    PLOG(ERROR) << "Failed to set TCP_KEEPALIVE on fd: " << fd;
+    return false;
+  }
 #endif

[Jana, 2015/04/06 23:20:00]

Should there be a #else? What are the platforms that are left out? FreeBSD
should have the same semantics as MACOSX (I think ... I'll take a look).

[mmenke, 2015/04/07 15:28:06]

There's also ChromeOS, but I believe that has OS_LINUX defined, given the huge
number of "#if defined(OS_LINUX) && !defined(OS_CHROMEOS)"

There's also OS_NACL, which I believe also has OS_LINUX set.

And then there's OS_FREEBSD, OS_SOLARIS, OS_QNX, OS_OPENBSD.  I think figuring
out which code each (unsupported) platform should use is beyond the scope of
this CL, particularly since we have no trybots, and I'm not going to try and
build them all manually.

For what it's worth, looks like FreeBSD should use the Linux path: 
http://svnweb.freebsd.org/base/head/sys/netinet/tcp.h?view=markup#l165

[Jana, 2015/04/07 19:47:19]

I think that the FBSD code is close to the MacOS code, but MacOS collapses some
of the options that FBSD needs explicit setting. See the setsockopt code 
for FBSD: http://fxr.watson.org/fxr/source/netinet/tcp_usrreq.c#L1492
and for MacOS:
http://fxr.watson.org/fxr/source/bsd/netinet/tcp_usrreq.c?v=xnu-2050.18.24#L1452

MacOS uses a single timer value for keepalives, when it's turned on, where FBSD
uses two different user defined values for the first keepalive and for intervals
between keepalives. That said, as you pointed out, this is not for this CL, so
I'll stop digging further :-)

   return true;
 }