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.h b/modules/audio_coding/acm2/acm_receiver.h
index 0731677..a2ae723 100644
--- a/modules/audio_coding/acm2/acm_receiver.h
+++ b/modules/audio_coding/acm2/acm_receiver.h
@@ -224,6 +224,7 @@
   //
   int DecoderByPayloadType(uint8_t payload_type,
                            CodecInst* codec) const;
+  absl::optional<SdpAudioFormat> DecoderByPayloadType(int payload_type) const;
 
   //
   // Enable NACK and set the maximum size of the NACK list. If NACK is already