Remove kVideoCodecUnknown from WebRTC.

There is no difference between how we handle "generic" and "unkown" codecs,
so we don't need to represent both.

Bug: webrtc:8136
Change-Id: I42b0dbc8a0bae67cc21742303c963c8dd5bde1f6
Reviewed-on: https://webrtc-review.googlesource.com/92086
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24316}
diff --git a/modules/video_coding/packet.cc b/modules/video_coding/packet.cc
index 5ffa45b..691ec62 100644
--- a/modules/video_coding/packet.cc
+++ b/modules/video_coding/packet.cc
@@ -26,7 +26,7 @@
       markerBit(false),
       timesNacked(-1),
       frameType(kEmptyFrame),
-      codec(kVideoCodecUnknown),
+      codec(kVideoCodecGeneric),
       is_first_packet_in_frame(false),
       completeNALU(kNaluUnset),
       insertStartCode(false),