Change WebRtcVideoReceiveChannel decoder_factory annotation

In a previous CL I annotated the decoder factory pointer as non-nullable
but it should be allowed to be null.

Bug: none
Change-Id: If1784aca43fceeefc30fce540f365e655cf5dde9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474362
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47773}

media/engine/webrtc_video_engine.cc

diff --git a/media/engine/webrtc_video_engine.cc b/media/engine/webrtc_video_engine.cc
index 08c9253..61824c8 100644
--- a/media/engine/webrtc_video_engine.cc
+++ b/media/engine/webrtc_video_engine.cc
@@ -136,7 +136,7 @@
 // should be advertised as supported.
 template <class T>
 std::vector<SdpVideoFormat> GetDefaultSupportedFormats(
-    const T* factory,
+    const T* absl_nullable factory,
     bool is_decoder_factory,
     const FieldTrialsView& trials) {
   if (!factory) {
@@ -203,7 +203,7 @@
 // where codecs are negotiated.
 template <class T>
 std::vector<Codec> GetPayloadTypesAndDefaultCodecs(
-    const T* factory,
+    const T* absl_nullable factory,
     bool is_decoder_factory,
     bool include_rtx,
     const FieldTrialsView& trials) {
@@ -2848,7 +2848,7 @@
     const MediaConfig& config,
     const VideoOptions& options,
     const CryptoOptions& crypto_options,
-    VideoDecoderFactory* absl_nonnull decoder_factory)
+    VideoDecoderFactory* absl_nullable decoder_factory)
     : MediaChannelUtil(call->network_thread(), config.enable_dscp),
       env_(env),
       network_thread_safety_(PendingTaskSafetyFlag::CreateAttachedToTaskQueue(

media/engine/webrtc_video_engine.h

diff --git a/media/engine/webrtc_video_engine.h b/media/engine/webrtc_video_engine.h
index d86596d4..57941e2 100644
--- a/media/engine/webrtc_video_engine.h
+++ b/media/engine/webrtc_video_engine.h
@@ -513,7 +513,7 @@
                             const MediaConfig& config,
                             const VideoOptions& options,
                             const CryptoOptions& crypto_options,
-                            VideoDecoderFactory* absl_nonnull decoder_factory);
+                            VideoDecoderFactory* absl_nullable decoder_factory);
   ~WebRtcVideoReceiveChannel() override;
 
  public: