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.cc b/modules/video_coding/decoder_database.cc
index 882de27..15d8dcc 100644
--- a/modules/video_coding/decoder_database.cc
+++ b/modules/video_coding/decoder_database.cc
@@ -19,13 +19,14 @@
   decoder_sequence_checker_.Detach();
 }
 
-bool VCMDecoderDataBase::DeregisterExternalDecoder(uint8_t payload_type) {
+VideoDecoder* VCMDecoderDataBase::DeregisterExternalDecoder(
+    uint8_t payload_type) {
   RTC_DCHECK_RUN_ON(&decoder_sequence_checker_);
   auto it = decoders_.find(payload_type);
   if (it == decoders_.end()) {
-    // Not found.
-    return false;
+    return nullptr;
   }
+
   // We can't use payload_type to check if the decoder is currently in use,
   // because payload type may be out of date (e.g. before we decode the first
   // frame after RegisterReceiveCodec).
@@ -33,8 +34,9 @@
     // Release it if it was registered and in use.
     current_decoder_ = absl::nullopt;
   }
+  VideoDecoder* ret = it->second;
   decoders_.erase(it);
-  return true;
+  return ret;
 }
 
 // Add the external decoder object to the list of external decoders.