Add GetContributionSources to TransformableIncomingAudioFrame

RTPHeader is not exported, so the TransformableIncomingAudioFrame can't be mocked in chrome tests, using a getter instead.

Bug: chromium:1247260
Change-Id: I2af4e6a88b3f4772b3bb50ee0ae9d5c80fed3ae4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/278785
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38352}
diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index de2c612..5efd3ea 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -73,6 +73,8 @@
   // information in the header as needed, for example to compile the list of
   // csrcs.
   virtual const RTPHeader& GetHeader() const = 0;
+
+  virtual rtc::ArrayView<const uint32_t> GetContributingSources() const = 0;
 };
 
 // Objects implement this interface to be notified with the transformed frame.