Add RTCStats for keyFramesEncoded, keyFramesDecoded.

This implements the correspondingly named JavaScript fields defined in
https://w3c.github.io/webrtc-stats/#inboundrtpstats-dict* and
https://w3c.github.io/webrtc-stats/#outboundrtpstats-dict*.

Bug: webrtc:7066
Change-Id: I431045bca80bf5faf27132c54f59c1f723c92952
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143683
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28404}
diff --git a/media/base/media_channel.h b/media/base/media_channel.h
index e98da00..8a97022 100644
--- a/media/base/media_channel.h
+++ b/media/base/media_channel.h
@@ -564,6 +564,7 @@
   int avg_encode_ms = 0;
   int encode_usage_percent = 0;
   uint32_t frames_encoded = 0;
+  uint32_t key_frames_encoded = 0;
   // https://w3c.github.io/webrtc-stats/#dom-rtcoutboundrtpstreamstats-totalencodetime
   uint64_t total_encode_time_ms = 0;
   // https://w3c.github.io/webrtc-stats/#dom-rtcoutboundrtpstreamstats-totalencodedbytestarget
@@ -597,6 +598,7 @@
   int framerate_render_output = 0;
   uint32_t frames_received = 0;
   uint32_t frames_decoded = 0;
+  uint32_t key_frames_decoded = 0;
   uint32_t frames_rendered = 0;
   absl::optional<uint64_t> qp_sum;
   int64_t interframe_delay_max_ms = -1;