WebRTC Opus C interface: Add support for non-48 kHz decode sample rate

Plus tests for 16 kHz.

Bug: webrtc:10631
Change-Id: I2d89bc6d0d9548f0ad7bb1e36d6dfde6b6b31f83
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138072
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28099}
diff --git a/modules/audio_coding/codecs/opus/opus_inst.h b/modules/audio_coding/codecs/opus/opus_inst.h
index 08c1e0f..9c3acb3 100644
--- a/modules/audio_coding/codecs/opus/opus_inst.h
+++ b/modules/audio_coding/codecs/opus/opus_inst.h
@@ -33,6 +33,7 @@
   int prev_decoded_samples;
   size_t channels;
   int in_dtx_mode;
+  int sample_rate_hz;
 };
 
 #endif  // MODULES_AUDIO_CODING_CODECS_OPUS_OPUS_INST_H_