The patch added in 78.0.3904.97-1 works. There is an extra step you must do to enable it though. You need to start Chromium with --ignore-gpu-blacklist flag. I have added it to my /etc/chromium.d/default-flags.
Also, if your GPU doesn't support vp9 video decoding, you can use h264ify or enhanced-h264ify browser extensions to force YouTube video player to use h264 video streams, as it uses vp9 by default.
