Reland "Fix definition of keyframes decoded statistics"

This is a reland of commit 0e37f5ebd44183d9fe5318d844235aae28fda86a
with backward compability added to allow downstream tests to migrate to the new signature.

Original change's description:
> Fix definition of keyframes decoded statistics
>
> which are defined to be measured after decoding, not before:
>   https://w3c.github.io/webrtc-stats/#dom-rtcinboundrtpstreamstats-keyframesdecoded
>
> BUG=webrtc:14728
>
> Change-Id: I0a83dde278e1ebe8acf787bdac729af369a1ecf8
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/315520
> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Commit-Queue: Philipp Hancke <phancke@microsoft.com>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#40545}

BUG=webrtc:14728

Change-Id: I4cf52bb22ba8244155b4fa8c367b9c0306a77590
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/316120
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#40553}
diff --git a/modules/video_coding/generic_decoder.h b/modules/video_coding/generic_decoder.h
index 7dc6d34..5922b52 100644
--- a/modules/video_coding/generic_decoder.h
+++ b/modules/video_coding/generic_decoder.h
@@ -46,6 +46,7 @@
   int64_t ntp_time_ms;
   RtpPacketInfos packet_infos;
   // ColorSpace is not stored here, as it might be modified by decoders.
+  VideoFrameType frame_type;
 };
 
 class VCMDecodedFrameCallback : public DecodedImageCallback {