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/generic_decoder.cc b/modules/video_coding/generic_decoder.cc
index 01500e7..eb16400 100644
--- a/modules/video_coding/generic_decoder.cc
+++ b/modules/video_coding/generic_decoder.cc
@@ -188,7 +188,7 @@
       _frameInfos(),
       _nextFrameInfoIdx(0),
       decoder_(decoder),
-      _codecType(kVideoCodecUnknown),
+      _codecType(kVideoCodecGeneric),
       _isExternal(isExternal),
       _last_keyframe_content_type(VideoContentType::UNSPECIFIED) {
   RTC_DCHECK(decoder_);