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.cc b/webrtc/call/flexfec_receive_stream_impl.cc
index a1e9bc6..16edadd 100644
--- a/webrtc/call/flexfec_receive_stream_impl.cc
+++ b/webrtc/call/flexfec_receive_stream_impl.cc
@@ -206,4 +206,9 @@
   return FlexfecReceiveStream::Stats();
 }
 
+const FlexfecReceiveStream::Config& FlexfecReceiveStreamImpl::GetConfig()
+    const {
+  return config_;
+}
+
 }  // namespace webrtc