Insert audio frame transformer between depacketizer and decoder.

The frame transformer is passed from RTPReceiverInterface through the
library to be eventually set in ChannelReceive, where the frame
transformation will occur in the follow-up CL.

Insertable Streams Web API explainer:
https://github.com/alvestrand/webrtc-media-streams/blob/master/explainer.md

Design doc for WebRTC library changes:
http://doc/1eiLkjNUkRy2FssCPLUp6eH08BZuXXoHfbbBP1ZN7EVk

Bug: webrtc:11380
Change-Id: I5af06d1431047ef50d00e304cf95e92a832b4220
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171872
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30956}

call/audio_receive_stream.h

@@ -21,6 +21,7 @@
 #include "api/call/transport.h"
 #include "api/crypto/crypto_options.h"
 #include "api/crypto/frame_decryptor_interface.h"
+#include "api/frame_transformer_interface.h"
 #include "api/rtp_parameters.h"
 #include "api/scoped_refptr.h"
 #include "api/transport/rtp/rtp_source.h"
@@ -150,6 +151,10 @@ class AudioReceiveStream {
     // decrypted in whatever way the caller choses. This is not required by
     // default.
     rtc::scoped_refptr<webrtc::FrameDecryptorInterface> frame_decryptor;
+
+    // An optional frame transformer used by insertable streams to transform
+    // encoded frames.
+    rtc::scoped_refptr<webrtc::FrameTransformerInterface> frame_transformer;
   };
 
   // Reconfigure the stream according to the Configuration.