Make CodecType conversion functions non-optional.

We can't handle no value here anyway and end up setting a default
at each call site. The defaults aren't even the same in each place.

BUG=None

Review-Url: https://codereview.webrtc.org/2998293002
Cr-Commit-Position: refs/heads/master@{#19485}
diff --git a/webrtc/modules/video_coding/packet.cc b/webrtc/modules/video_coding/packet.cc
index 22ef7ae..7bc4d94 100644
--- a/webrtc/modules/video_coding/packet.cc
+++ b/webrtc/modules/video_coding/packet.cc
@@ -134,6 +134,8 @@
       codec = kVideoCodecH264;
       return;
     case kRtpVideoGeneric:
+      codec = kVideoCodecGeneric;
+      return;
     case kRtpVideoNone:
       codec = kVideoCodecUnknown;
       return;