AcmReceiver: Eliminate AcmReceiver::decoders_

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2351183002
Cr-Commit-Position: refs/heads/master@{#14335}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.h b/webrtc/modules/audio_coding/neteq/neteq_impl.h
index 7903ba6..dd35301 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.h
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.h
@@ -173,6 +173,8 @@
 
   rtc::Optional<CodecInst> GetDecoder(int payload_type) const override;
 
+  const SdpAudioFormat* GetDecoderFormat(int payload_type) const override;
+
   int SetTargetNumberOfChannels() override;
 
   int SetTargetSampleRate() override;