Get rid of unnecessary cast of FlexfecReceiveStreamImpl to FlexfecReceiveStream

webrtc/call/flexfec_receive_stream.h

Index: webrtc/call/flexfec_receive_stream.h
diff --git a/webrtc/call/flexfec_receive_stream.h b/webrtc/call/flexfec_receive_stream.h
index babeee0ecb64c4c93d4c961c9c3adaeb100893fb..353811a92262924481d899a8afa0791e6436210f 100644
--- a/webrtc/call/flexfec_receive_stream.h
+++ b/webrtc/call/flexfec_receive_stream.h
@@ -23,6 +23,8 @@ namespace webrtc {
 
 class FlexfecReceiveStream {
  public:
+  virtual ~FlexfecReceiveStream() = default;
+
   struct Stats {
     std::string ToString(int64_t time_ms) const;
 
@@ -82,8 +84,7 @@ class FlexfecReceiveStream {
 
   virtual Stats GetStats() const = 0;
 
- protected:
-  virtual ~FlexfecReceiveStream() = default;
+  virtual const Config& GetConfig() const = 0;
 };
 
 }  // namespace webrtc