Use NullSocketServer instead of PhysicalSocketServer in SignalThread

BUG=webrtc:6125

Review-Url: https://codereview.webrtc.org/2164333002
Cr-Commit-Position: refs/heads/master@{#13525}

webrtc/base/nethelpers.cc

@@ -82,8 +82,8 @@ int ResolveHostname(const std::string& hostname, int family,
 }
 
 // AsyncResolver
-AsyncResolver::AsyncResolver() : error_(-1) {
-}
+AsyncResolver::AsyncResolver()
+    : SignalThread(false /* use_socket_server */), error_(-1) {}
 
 AsyncResolver::~AsyncResolver() = default;
 

webrtc/base/signalthread.cc

@@ -18,9 +18,9 @@ namespace rtc {
 // SignalThread
 ///////////////////////////////////////////////////////////////////////////////
 
-SignalThread::SignalThread()
+SignalThread::SignalThread(bool use_socket_server)
     : main_(Thread::Current()),
-      worker_(this),
+      worker_(this, use_socket_server),
       state_(kInit),
       refcount_(1) {
   main_->SignalQueueDestroyed.connect(this,

webrtc/base/signalthread.h

@@ -14,6 +14,7 @@
 #include <string>
 
 #include "webrtc/base/constructormagic.h"
+#include "webrtc/base/nullsocketserver.h"
 #include "webrtc/base/sigslot.h"
 #include "webrtc/base/thread.h"
 
@@ -40,7 +41,7 @@ class SignalThread
     : public sigslot::has_slots<>,
       protected MessageHandler {
  public:
-  SignalThread();
+  explicit SignalThread(bool use_socket_server = true);
 
   // Context: Main Thread.  Call before Start to change the worker's name.
   bool SetName(const std::string& name, const void* obj);
@@ -102,7 +103,11 @@ class SignalThread
 
   class Worker : public Thread {
    public:
-    explicit Worker(SignalThread* parent) : parent_(parent) {}
+    explicit Worker(SignalThread* parent, bool use_socket_server)
+        : Thread(use_socket_server
+                     ? SocketServer::CreateDefault()
+                     : std::unique_ptr<SocketServer>(new NullSocketServer())),
+          parent_(parent) {}
     ~Worker() override;
     void Run() override;