message: Add method to get FD for physical socket
  Cr-Commit-Position: refs/heads/main@{#39234}
diff: @@ -173,6 +173,8 @@ class PhysicalSocket : public Socket, public sigslot::has_slots<> {
  SocketServer* socketserver() { return ss_; }
  + SOCKET GetSocketFD() const { return s_; }
  +
  protected:
  int DoConnect(const SocketAddress& connect_addr);
  