Clean up FlexfecReceiveStream ctor signatures.

BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2535173008
Cr-Commit-Position: refs/heads/master@{#15476}
diff --git a/webrtc/call/call.h b/webrtc/call/call.h
index 34f6833..a9de18d 100644
--- a/webrtc/call/call.h
+++ b/webrtc/call/call.h
@@ -120,7 +120,7 @@
       VideoReceiveStream* receive_stream) = 0;
 
   virtual FlexfecReceiveStream* CreateFlexfecReceiveStream(
-      FlexfecReceiveStream::Config configuration) = 0;
+      const FlexfecReceiveStream::Config& config) = 0;
   virtual void DestroyFlexfecReceiveStream(
       FlexfecReceiveStream* receive_stream) = 0;