Fix and improve FlexFEC configuration for RTP/RTCP.

Fix: Order of assignments is now correct, after being incorrect
due to an incorrect merge between
https://codereview.webrtc.org/2617373002/ and
https://codereview.webrtc.org/2589713003.

Improvement: Set parameters in more places, allowing for
correct reconfiguration. Add TODOs to point of minor issues
with current configuration.

TESTED=By locally patching an application using this code.
BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2660403004
Cr-Commit-Position: refs/heads/master@{#16431}

webrtc/media/engine/fakewebrtccall.h

diff --git a/webrtc/media/engine/fakewebrtccall.h b/webrtc/media/engine/fakewebrtccall.h
index 9ceda0f..8557720 100644
--- a/webrtc/media/engine/fakewebrtccall.h
+++ b/webrtc/media/engine/fakewebrtccall.h
@@ -174,7 +174,7 @@
  public:
   explicit FakeVideoReceiveStream(webrtc::VideoReceiveStream::Config config);
 
-  const webrtc::VideoReceiveStream::Config& GetConfig();
+  const webrtc::VideoReceiveStream::Config& GetConfig() const;
 
   bool IsReceiving() const;