NetEq: Add new method last_output_sample_rate_hz

This change moves the logics for keeping track of the last ouput
sample rate from AcmReceiver to NetEq, where it fits better. The
getter function AcmReceiver::current_sample_rate_hz() is renamed to
last_output_sample_rate_hz().

BUG=webrtc:3520

Review URL: https://codereview.webrtc.org/1467163002

Cr-Commit-Position: refs/heads/master@{#10754}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.h b/webrtc/modules/audio_coding/neteq/neteq_impl.h
index 60c846d..0fc204f 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.h
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.h
@@ -168,6 +168,8 @@
 
   bool GetPlayoutTimestamp(uint32_t* timestamp) override;
 
+  int last_output_sample_rate_hz() const override;
+
   int SetTargetNumberOfChannels() override;
 
   int SetTargetSampleRate() override;
@@ -375,6 +377,7 @@
   StatisticsCalculator stats_ GUARDED_BY(crit_sect_);
   int fs_hz_ GUARDED_BY(crit_sect_);
   int fs_mult_ GUARDED_BY(crit_sect_);
+  int last_output_sample_rate_hz_ GUARDED_BY(crit_sect_);
   size_t output_size_samples_ GUARDED_BY(crit_sect_);
   size_t decoder_frame_length_ GUARDED_BY(crit_sect_);
   Modes last_mode_ GUARDED_BY(crit_sect_);