Let NetEq use the PLC output from a decoder

This change enables NetEq to use the packet concealment audio (aka
PLC) produced by a decoder. The change also includes a new API to the
AudioDecoder interface, which lets the decoder implementation generate
and deliver concealment audio.

Bug: webrtc:9180
Change-Id: Icaacebccf645d4694b0d2d6310f6f2c7132881c4
Reviewed-on: https://webrtc-review.googlesource.com/96340
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24738}
diff --git a/modules/audio_coding/neteq/defines.h b/modules/audio_coding/neteq/defines.h
index 768f0b9..46926fa 100644
--- a/modules/audio_coding/neteq/defines.h
+++ b/modules/audio_coding/neteq/defines.h
@@ -39,6 +39,7 @@
   kModePreemptiveExpandFail,
   kModeRfc3389Cng,
   kModeCodecInternalCng,
+  kModeCodecPlc,
   kModeDtmf,
   kModeError,
   kModeUndefined = -1