Move decoder instance ownership to VideoReceiver2

This moves the ownership away from VideoReceiveStream2 and closer to
VCMDecoderDataBase. That facilitates unregistration (upcoming change)
without recreating receive streams.

Bug: none
Change-Id: I812175134730a0ffbf7077fd149c8489481c73d8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272481
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37866}
diff --git a/modules/video_coding/decoder_database.h b/modules/video_coding/decoder_database.h
index 98bbf85..59b683b 100644
--- a/modules/video_coding/decoder_database.h
+++ b/modules/video_coding/decoder_database.h
@@ -30,7 +30,9 @@
   VCMDecoderDataBase& operator=(const VCMDecoderDataBase&) = delete;
   ~VCMDecoderDataBase() = default;
 
-  bool DeregisterExternalDecoder(uint8_t payload_type);
+  // Returns a pointer to the previously registered decoder or nullptr if none
+  // was registered for the `payload_type`.
+  VideoDecoder* DeregisterExternalDecoder(uint8_t payload_type);
   void RegisterExternalDecoder(uint8_t payload_type,
                                VideoDecoder* external_decoder);
   bool IsExternalDecoderRegistered(uint8_t payload_type) const;