Do not register VideoSendStream to PacketFeedbackObserver when the current FEC controller do not use Loss Vector Mask.

Bug: chromium:804865
Change-Id: Ib197ff05266a20b8358724e7d8bfe2b085a2de23
Reviewed-on: https://webrtc-review.googlesource.com/43123
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Ying Wang <yinwa@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21742}

modules/video_coding/fec_controller_default.h

@@ -40,6 +40,7 @@ class FecControllerDefault : public FecController {
                           std::vector<bool> loss_mask_vector,
                           int64_t round_trip_time_ms) override;
   void UpdateWithEncodedData(const EncodedImage& encoded_image) override;
+  bool UseLossVectorMask() override { return false; }
 
  private:
   enum { kBitrateAverageWinMs = 1000 };