[Stats] Populate "frames" stats for video source.

Spec: https://www.w3.org/TR/webrtc-stats/#dom-rtcvideosourcestats-frames

Wiring up the "frames" stats with the cumulative fps counter on the video source.

Tests:
./out/Default/peerconnection_unittests
./out/Default/video_engine_tests

Bug: webrtc:12499
Change-Id: I4103f56ed04cb464f5f7e70fbf2d77c25a867a68
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208782
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33404}
diff --git a/pc/rtc_stats_collector.cc b/pc/rtc_stats_collector.cc
index 6cb1dcc..122ae9f 100644
--- a/pc/rtc_stats_collector.cc
+++ b/pc/rtc_stats_collector.cc
@@ -1620,6 +1620,7 @@
           if (video_sender_info) {
             video_source_stats->frames_per_second =
                 video_sender_info->framerate_input;
+            video_source_stats->frames = video_sender_info->frames;
           }
         }
         media_source_stats = std::move(video_source_stats);