Demonstrates that MediaStreamVideoTrackSource produces a
VFR MP4 when recording a static screen. Chrome throttles frame delivery
for power savings, resulting in huge timestamp gaps.
Verify with ffprobe:
ffprobe -v error -select_streams v:0 \
-show_entries stream=nb_frames,duration,r_frame_rate,avg_frame_rate \
-of default=noprint_wrappers=1 recording.mp4