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

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

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2339953002
Cr-Commit-Position: refs/heads/master@{#14274}

webrtc/modules/audio_coding/neteq/include/neteq.h

@@ -252,6 +252,8 @@ class NetEq {
   // (Config::sample_rate_hz) is returned.
   virtual int last_output_sample_rate_hz() const = 0;
 
+  virtual rtc::Optional<CodecInst> GetDecoder(int payload_type) const = 0;
+
   // Not implemented.
   virtual int SetTargetNumberOfChannels() = 0;