WebRTC-DeprecateGlobalFieldTrialString/Enabled/ - part 5/inf

Add field trials to audio api.

It is added as a pointer with nullptr as default.
It is not (yet) used anywhere.
Usage of field trials comes in subsequent patches.

Bug: webrtc:10335
Change-Id: Icbe22d95c356a6fefde34590f11ea63f005ab09e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/255521
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36213}
diff --git a/api/audio_codecs/opus/audio_decoder_opus.h b/api/audio_codecs/opus/audio_decoder_opus.h
index ec0f61d..4e44309 100644
--- a/api/audio_codecs/opus/audio_decoder_opus.h
+++ b/api/audio_codecs/opus/audio_decoder_opus.h
@@ -18,6 +18,7 @@
 #include "api/audio_codecs/audio_codec_pair_id.h"
 #include "api/audio_codecs/audio_decoder.h"
 #include "api/audio_codecs/audio_format.h"
+#include "api/webrtc_key_value_config.h"
 #include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
@@ -34,7 +35,8 @@
   static void AppendSupportedDecoders(std::vector<AudioCodecSpec>* specs);
   static std::unique_ptr<AudioDecoder> MakeAudioDecoder(
       Config config,
-      absl::optional<AudioCodecPairId> codec_pair_id = absl::nullopt);
+      absl::optional<AudioCodecPairId> codec_pair_id = absl::nullopt,
+      const WebRtcKeyValueConfig* field_trials = nullptr);
 };
 
 }  // namespace webrtc