Define FecControllerOverride and plumb it down to VideoEncoder

The purpose of this interface is to allow VideoEncoder to override
the bandwidth allocation set by FecController in RtpVideoSender.

This CL defines the interface and sends it down to VideoSender.
Two upcoming CLs will:
1. Make LibvpxVp8Encoder pass it on to the (injectable)
   FrameBufferController, where it might be put to good use.
2. Modify RtpVideoSender to respond to the message sent to it
   via this API.

TBR=kwiberg@webrtc.org

Bug: webrtc:10769
Change-Id: I2ef82f0ddcde7fd078e32d8aabf6efe43e0f7f8a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143962
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28416}

test/video_encoder_proxy_factory.h

@@ -65,22 +65,32 @@ class VideoEncoderProxyFactory final : public VideoEncoderFactory {
     explicit EncoderProxy(VideoEncoder* encoder) : encoder_(encoder) {}
 
    private:
+    void SetFecControllerOverride(
+        FecControllerOverride* fec_controller_override) override {
+      encoder_->SetFecControllerOverride(fec_controller_override);
+    }
+
     int32_t Encode(const VideoFrame& input_image,
                    const std::vector<VideoFrameType>* frame_types) override {
       return encoder_->Encode(input_image, frame_types);
     }
+
     int32_t InitEncode(const VideoCodec* config,
                        const Settings& settings) override {
       return encoder_->InitEncode(config, settings);
     }
+
     int32_t RegisterEncodeCompleteCallback(
         EncodedImageCallback* callback) override {
       return encoder_->RegisterEncodeCompleteCallback(callback);
     }
+
     int32_t Release() override { return encoder_->Release(); }
+
     void SetRates(const RateControlParameters& parameters) override {
       encoder_->SetRates(parameters);
     }
+
     VideoEncoder::EncoderInfo GetEncoderInfo() const override {
       return encoder_->GetEncoderInfo();
     }