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}
diff --git a/api/video_codecs/video_encoder.cc b/api/video_codecs/video_encoder.cc
index ffdcf8b..d3f16a0 100644
--- a/api/video_codecs/video_encoder.cc
+++ b/api/video_codecs/video_encoder.cc
@@ -118,6 +118,9 @@
 
 VideoEncoder::RateControlParameters::~RateControlParameters() = default;
 
+void VideoEncoder::SetFecControllerOverride(
+    FecControllerOverride* fec_controller_override) {}
+
 int32_t VideoEncoder::InitEncode(const VideoCodec* codec_settings,
                                  int32_t number_of_cores,
                                  size_t max_payload_size) {