Move class VideoCodec from common_types.h to its own api header file.

Bug: webrtc:7660
Change-Id: I91f19bfc2565461328f30081f8383e136419aefb
Reviewed-on: https://webrtc-review.googlesource.com/79881
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23544}

modules/rtp_rtcp/include/rtp_payload_registry.h

@@ -16,13 +16,12 @@
 
 #include "api/audio_codecs/audio_format.h"
 #include "api/optional.h"
+#include "api/video_codecs/video_codec.h"
 #include "modules/rtp_rtcp/source/rtp_utility.h"
 #include "rtc_base/criticalsection.h"
 
 namespace webrtc {
 
-class VideoCodec;
-
 class RTPPayloadRegistry {
  public:
   RTPPayloadRegistry();