Use NullSocketServer instead of PhysicalSocketServer in SignalThread

webrtc/base/nethelpers.cc

Index: webrtc/base/nethelpers.cc
diff --git a/webrtc/base/nethelpers.cc b/webrtc/base/nethelpers.cc
index 1f6bb8df7b7f71628524df9bd1fa27240ab9fe7b..4bcf5b60e6988af6d58d658d1c06b98141232625 100644
--- a/webrtc/base/nethelpers.cc
+++ b/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;