Voe::Channel: Turned GetPlayoutFrequency into GetRtpTimestampRateHz.

This gets rid of a bit of codec-specific code in VoE.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2355483003
Cr-Commit-Position: refs/heads/master@{#14614}
diff --git a/webrtc/modules/audio_coding/acm2/acm_receiver.h b/webrtc/modules/audio_coding/acm2/acm_receiver.h
index a9550fb..6415074 100644
--- a/webrtc/modules/audio_coding/acm2/acm_receiver.h
+++ b/webrtc/modules/audio_coding/acm2/acm_receiver.h
@@ -209,6 +209,8 @@
   //
   int LastAudioCodec(CodecInst* codec) const;
 
+  rtc::Optional<SdpAudioFormat> LastAudioFormat() const;
+
   //
   // Get a decoder given its registered payload-type.
   //
@@ -273,6 +275,7 @@
 
   rtc::CriticalSection crit_sect_;
   rtc::Optional<CodecInst> last_audio_decoder_ GUARDED_BY(crit_sect_);
+  rtc::Optional<SdpAudioFormat> last_audio_format_ GUARDED_BY(crit_sect_);
   ACMResampler resampler_ GUARDED_BY(crit_sect_);
   std::unique_ptr<int16_t[]> last_audio_buffer_ GUARDED_BY(crit_sect_);
   CallStatistics call_stats_ GUARDED_BY(crit_sect_);