Replace sigslot usages with callback list library.

- Replace few sigslot usages in jsep_transport_controller.
- There is still one sigslot usages in this file so keeping the inheritance
and that is the reason for not having a binary size gain in this CL.
- Remaining sigslot will be removed in a separate CL.

Bug: webrtc:11943
Change-Id: Idb8fa1090b037c48eeb62f54cffd3c485cebfcda
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/190146
Reviewed-by: Andrey Logvin <landrey@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Lahiru Ginnaliya Gamathige <glahiru@webrtc.org>
Commit-Queue: Lahiru Ginnaliya Gamathige <glahiru@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33034}

test/peer_scenario/scenario_connection.cc

@@ -165,8 +165,12 @@ void ScenarioIceConnectionImpl::SetRemoteSdp(SdpType type,
                                              const std::string& remote_sdp) {
   RTC_DCHECK_RUN_ON(signaling_thread_);
   remote_description_ = webrtc::CreateSessionDescription(type, remote_sdp);
-  jsep_controller_->SignalIceCandidatesGathered.connect(
-      this, &ScenarioIceConnectionImpl::OnCandidates);
+  jsep_controller_->SubscribeIceCandidateGathered(
+      [this](const std::string& transport,
+             const std::vector<cricket::Candidate>& candidate) {
+        ScenarioIceConnectionImpl::OnCandidates(transport, candidate);
+      });
+
   auto res = jsep_controller_->SetRemoteDescription(
       remote_description_->GetType(), remote_description_->description());
   RTC_CHECK(res.ok()) << res.message();