Pass FecControllerOverride to Vp8FrameBufferControllerFactory::Create

Previously, FecControllerOverride was passed to
Vp8FrameBufferController::SetFecControllerOverride. Passing to
the factory is a more elegant way, since it's only used when
the controller is constructed.

TBR=kwiberg@webrtc.org

Bug: webrtc:10769
Change-Id: Iae599889e7ca9003e3200c2911239cbb763ee65a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144380
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28443}

modules/video_coding/codecs/vp8/libvpx_vp8_encoder.h

@@ -141,6 +141,8 @@ class LibvpxVp8Encoder : public VideoEncoder {
       std::string group_name);
   FramerateController framerate_controller_;
   int num_steady_state_frames_;
+
+  FecControllerOverride* fec_controller_override_;
 };
 
 }  // namespace webrtc