AcmReceiver: Look up last decoder in NetEq's table of decoders

AcmReceiver::decoders_ is now one step closer to being unused.

(This is a re-land of https://codereview.webrtc.org/2339953002.)

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2354453003
Cr-Commit-Position: refs/heads/master@{#14303}

webrtc/modules/audio_coding/neteq/neteq_impl.h

@@ -169,6 +169,8 @@ class NetEqImpl : public webrtc::NetEq {
 
   int last_output_sample_rate_hz() const override;
 
+  rtc::Optional<CodecInst> GetDecoder(int payload_type) const override;
+
   int SetTargetNumberOfChannels() override;
 
   int SetTargetSampleRate() override;