Adds clearer function to create rtc::Thread without Physical SocketServer

Moves logic of default SocketServer from MessageQueue to SocketServer

Review-Url: https://codereview.webrtc.org/1891293002
Cr-Commit-Position: refs/heads/master@{#12541}

webrtc/base/socketserver.h

@@ -11,6 +11,7 @@
 #ifndef WEBRTC_BASE_SOCKETSERVER_H_
 #define WEBRTC_BASE_SOCKETSERVER_H_
 
+#include <memory>
 #include "webrtc/base/socketfactory.h"
 
 namespace rtc {
@@ -27,6 +28,7 @@ class SocketServer : public SocketFactory {
  public:
   static const int kForever = -1;
 
+  static std::unique_ptr<SocketServer> CreateDefault();
   // When the socket server is installed into a Thread, this function is
   // called to allow the socket server to use the thread's message queue for
   // any messaging that it might need to perform.