Add new codec for FlexFEC.

This CL does nothing except adding new strings and enums corresponding to
the new codec.

R=stefan@webrtc.org
BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2470103002
Cr-Commit-Position: refs/heads/master@{#14943}
diff --git a/webrtc/common_types.h b/webrtc/common_types.h
index 678b856..24ba00a 100644
--- a/webrtc/common_types.h
+++ b/webrtc/common_types.h
@@ -558,6 +558,7 @@
   kVideoCodecI420,
   kVideoCodecRED,
   kVideoCodecULPFEC,
+  kVideoCodecFlexfec,
   kVideoCodecGeneric,
   kVideoCodecUnknown
 };