Add method to get FD for physical socket

Bug: b/264850454, webrtc:14866
Change-Id: I8a47aa3560fa8a5d20a9984cc507c543256788dd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/291521
Reviewed-by: Victor Boivie <boivie@google.com>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39234}

rtc_base/physical_socket_server.h

@@ -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);