Reland "Enable using a custom NetEqFactory in simulations"

This is a reland of 2a11b2451a4068746fa0c55fa210efd4a15e4423
There are no changes compared to the first attempt.

Original change's description:
> Enable using a custom NetEqFactory in simulations
>
> Bug: webrtc:11005
> Change-Id: I8a15f77953cbd3c29a75c7cfc77f926b138994b9
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165580
> Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#30286}

TBR=kwiberg

Bug: webrtc:11005
Change-Id: I4aa377e05916bd23f8f63aece9d0e27731c80d3e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/166465
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30319}
diff --git a/api/test/neteq_simulator_factory.h b/api/test/neteq_simulator_factory.h
index 3c0cbe0..af747f6 100644
--- a/api/test/neteq_simulator_factory.h
+++ b/api/test/neteq_simulator_factory.h
@@ -15,6 +15,7 @@
 #include <string>
 
 #include "absl/strings/string_view.h"
+#include "api/neteq/neteq_factory.h"
 #include "api/test/neteq_simulator.h"
 
 namespace webrtc {
@@ -40,6 +41,8 @@
     int skip_get_audio_events = 0;
     // A WebRTC field trial string to be used during the simulation.
     std::string field_trial_string;
+    // A custom NetEqFactory can be used.
+    NetEqFactory* neteq_factory = nullptr;
   };
   // This function takes the same arguments as the neteq_rtpplay utility.
   std::unique_ptr<NetEqSimulator> CreateSimulator(int argc, char* argv[]);