Deprecate GetTimestamp in TransformableFrameInterface.

Bug: webrtc:526671875
Change-Id: If7e041cd70542fc3e11c9036dcc0d604aeb80632
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/487340
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48160}

api/frame_transformer_interface.h

diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index 5dab9c1..f29fc38 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -72,7 +72,7 @@
   // timestamp is not known. Use GetRtpTimestampInfo instead, which makes it
   // possible to make a distinction between these two cases.
   // TODO(https://bugs.webrtc.org/526671875): Deprecate and remove this method.
-  // [[deprecated("Use GetRtpTimestampInfo instead")]]
+  [[deprecated("Use GetRtpTimestampInfo instead")]]
   virtual uint32_t GetTimestamp() const = 0;
   virtual void SetRTPTimestamp(uint32_t rtp_timestamp_with_offset) = 0;