Make AudioFrameType an enum class, and move to audio_coding_module_typedefs.h

Bug: webrtc:5876
Change-Id: I0c92f9410fcf0832bfa321229b3437134255dba6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128085
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27190}

modules/audio_coding/include/audio_coding_module_typedefs.h

@@ -33,6 +33,12 @@ enum ACMVADMode {
   VADVeryAggr = 3
 };
 
+enum class AudioFrameType {
+  kEmptyFrame = 0,
+  kAudioFrameSpeech = 1,
+  kAudioFrameCN = 2,
+};
+
 ///////////////////////////////////////////////////////////////////////////
 //
 // Enumeration of Opus mode for intended application.