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}

modules/video_coding/include/video_codec_interface.h

@@ -83,7 +83,7 @@ union CodecSpecificInfoUnion {
 // must be fitted with a copy-constructor. This is because it is copied
 // in the copy-constructor of VCMEncodedFrame.
 struct CodecSpecificInfo {
-  CodecSpecificInfo() : codecType(kVideoCodecUnknown), codec_name(nullptr) {
+  CodecSpecificInfo() : codecType(kVideoCodecGeneric), codec_name(nullptr) {
     memset(&codecSpecific, 0, sizeof(codecSpecific));
   }
   VideoCodecType codecType;