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}

api/fec_controller.h

@@ -74,6 +74,9 @@ class FecController {
 
   // Informs of encoded output.
   virtual void UpdateWithEncodedData(const EncodedImage& encoded_image) = 0;
+
+  // Returns whether this FEC Controller needs Loss Vector Mask as input.
+  virtual bool UseLossVectorMask() = 0;
 };
 
 }  // namespace webrtc