Deprecate encoded audio frame GetHeader

Bug: chromium:1456628
Change-Id: Ifc7d1aa1153c0593c673381f153e5793b94c98c3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/310420
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Tony Herre <herre@google.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40359}

api/frame_transformer_interface.h

diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index 47058d5..73d2702 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -69,12 +69,10 @@
   virtual ~TransformableAudioFrameInterface() = default;
 
   virtual void SetRTPTimestamp(uint32_t timestamp) = 0;
-  // Exposes the frame header, enabling the interface clients to use the
-  // information in the header as needed, for example to compile the list of
-  // csrcs.
-  // TODO(crbug.com/1453226): Deprecate and remove once callers have migrated to
-  // the getters for specific fields.
-  virtual const RTPHeader& GetHeader() const = 0;
+
+  // TODO(crbug.com/1453226): Remove after a few weeks.
+  [[deprecated("Use specific getters instead.")]] virtual const RTPHeader&
+  GetHeader() const = 0;
 
   virtual rtc::ArrayView<const uint32_t> GetContributingSources() const = 0;