Inform VideoEncoder of negotiated capabilities

After this CL lands, an announcement will be made to
discuss-webrtc about the deprecation of one version
of InitEncode().

Bug: webrtc:10720
Change-Id: Ib992af0272bbb16ae16ef7e69491f365702d179e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/140884
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28224}

modules/video_coding/codecs/multiplex/include/multiplex_encoder_adapter.h

@@ -42,6 +42,8 @@ class MultiplexEncoderAdapter : public VideoEncoder {
   int InitEncode(const VideoCodec* inst,
                  int number_of_cores,
                  size_t max_payload_size) override;
+  int InitEncode(const VideoCodec* inst,
+                 const VideoEncoder::Settings& settings) override;
   int Encode(const VideoFrame& input_image,
              const std::vector<VideoFrameType>* frame_types) override;
   int RegisterEncodeCompleteCallback(EncodedImageCallback* callback) override;