acm_receiver_unittest: Don't rely on the ACM to create encoders

It will soon lose the ability to do so.

Bug: webrtc:8396
Change-Id: Ifca101fce0c349dba8c402ab2b6ad614061a88f6
Reviewed-on: https://webrtc-review.googlesource.com/101281
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24836}
diff --git a/modules/audio_coding/acm2/acm_receiver.cc b/modules/audio_coding/acm2/acm_receiver.cc
index 892abe5..f631746 100644
--- a/modules/audio_coding/acm2/acm_receiver.cc
+++ b/modules/audio_coding/acm2/acm_receiver.cc
@@ -361,6 +361,12 @@
   }
 }
 
+absl::optional<SdpAudioFormat> AcmReceiver::DecoderByPayloadType(
+    int payload_type) const {
+  rtc::CritScope lock(&crit_sect_);
+  return neteq_->GetDecoderFormat(payload_type);
+}
+
 int AcmReceiver::EnableNack(size_t max_nack_list_size) {
   neteq_->EnableNack(max_nack_list_size);
   return 0;