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

Plus tests fo 16 kHz.

Bug: webrtc:10631
Change-Id: I162c40b6120d7e308e535faba7501e437b0b5dc4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/137047
Reviewed-by: Minyue Li <minyue@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28029}
diff --git a/modules/audio_coding/codecs/opus/opus_interface.h b/modules/audio_coding/codecs/opus/opus_interface.h
index f5bcc45..54ecadd 100644
--- a/modules/audio_coding/codecs/opus/opus_interface.h
+++ b/modules/audio_coding/codecs/opus/opus_interface.h
@@ -35,6 +35,7 @@
  *                                 Favor speech intelligibility.
  *                             1 - Audio applications.
  *                                 Favor faithfulness to the original input.
+ *      - sample_rate_hz     : sample rate of input audio
  *
  * Output:
  *      - inst               : a pointer to Encoder context that is created
@@ -45,7 +46,8 @@
  */
 int16_t WebRtcOpus_EncoderCreate(OpusEncInst** inst,
                                  size_t channels,
-                                 int32_t application);
+                                 int32_t application,
+                                 int sample_rate_hz);
 
 /****************************************************************************
  * WebRtcOpus_MultistreamEncoderCreate(...)