Add support to not use turn server as stun server.

If a stun server is already there, the benefit of adding turn servers as stun servers is small,
and it may create unnecessary stun candidates.

Bug: webrtc:11059
Change-Id: Ia37b43b787180af4d91c1c07c866ccbf1db80262
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158680
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29644}

p2p/client/basic_port_allocator.h

@@ -290,6 +290,7 @@ struct RTC_EXPORT PortConfiguration : public rtc::MessageData {
   ServerAddresses stun_servers;
   std::string username;
   std::string password;
+  bool use_turn_server_as_stun_server_disabled = false;
 
   typedef std::vector<RelayServerConfig> RelayList;
   RelayList relays;