Get rid of unnecessary cast of FlexfecReceiveStreamImpl to FlexfecReceiveStream

BUG=None

Review-Url: https://codereview.webrtc.org/2967913002
Cr-Commit-Position: refs/heads/master@{#19131}
diff --git a/webrtc/call/flexfec_receive_stream_impl.h b/webrtc/call/flexfec_receive_stream_impl.h
index 81c09d6..de5de41 100644
--- a/webrtc/call/flexfec_receive_stream_impl.h
+++ b/webrtc/call/flexfec_receive_stream_impl.h
@@ -40,8 +40,6 @@
       ProcessThread* process_thread);
   ~FlexfecReceiveStreamImpl() override;
 
-  const Config& GetConfig() const { return config_; }
-
   // RtpPacketSinkInterface.
   void OnRtpPacket(const RtpPacketReceived& packet) override;
 
@@ -49,6 +47,7 @@
   void Start() override;
   void Stop() override;
   Stats GetStats() const override;
+  const Config& GetConfig() const override;
 
  private:
   // Config.