Deprecate TransformableVideoFrame GetAdditionalData

It's unused in Chromium and internally - GetMetadata() provides
sufficient information.

Bug: chromium:1414370
Change-Id: Id93bdccbda85090c1aa2fabf5d6b7b79f2b1e2e6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/292862
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Tony Herre <herre@google.com>
Commit-Queue: Tony Herre <herre@google.com>
Cr-Commit-Position: refs/heads/main@{#39287}
diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index fba09d5..92832c0 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -57,9 +57,9 @@
   // when the transformation applied to the frame is encryption/decryption, the
   // additional data holds the serialized generic frame descriptor extension
   // calculated in webrtc::RtpDescriptorAuthentication.
-  // TODO(bugs.webrtc.org/11380) remove from interface once
-  // webrtc::RtpDescriptorAuthentication is exposed in api/.
-  virtual std::vector<uint8_t> GetAdditionalData() const = 0;
+  // This has been superseeded by GetMetadata() and will be removed shortly.
+  [[deprecated("https://crbug.com/1414370")]] virtual std::vector<uint8_t>
+  GetAdditionalData() const = 0;
 
   virtual const VideoFrameMetadata& GetMetadata() const = 0;
   // TODO(https://crbug.com/webrtc/14709): Make pure virtual when Chromium MOCK