Add a NetLog event for when sockets are closed.

net/socket/tcp_socket_win.cc

Index: net/socket/tcp_socket_win.cc
diff --git a/net/socket/tcp_socket_win.cc b/net/socket/tcp_socket_win.cc
index 4667bd8e423dd5b034969c715569fbf8843085c1..88db36fd41c3da42f24b7eb17da0bab4d6a7ac52 100644
--- a/net/socket/tcp_socket_win.cc
+++ b/net/socket/tcp_socket_win.cc
@@ -641,6 +641,9 @@ void TCPSocketWin::Close() {
   DCHECK(CalledOnValidThread());
 
   if (socket_ != INVALID_SOCKET) {
+    // Only log the close event if there's actually a socket to close.
+    net_log_.AddEvent(NetLog::EventType::TYPE_SOCKET_CLOSED);
+
     // Note: don't use CancelIo to cancel pending IO because it doesn't work
     // when there is a Winsock layered service provider.