Do we still want to do this?

Definitely want to do some testing to see if it makes a difference. 

Do y'all have traces where the media thread is overloaded and smoothness ends up being impacted? I'm just trying to understand what the underlying data motivating this change is.

I support any request for data to verify my suggestion. Probably that means implementing this and checking the AV-Analysis + YouTube metrics.

The reason I suggested this though: It's not necessary for it to be overloaded, just for there to be long blocking tasks on it -- of which software video decode may definitely block 10s to 100s of ms (audio decode is also on this thread, but is generally very fast). The media thread does not run with the same thread priorities as the previous compositor thread either.

Ok, software video decode as a use case makes sense here.  If you're adding more work to a thread, then maybe that would push work past some threshold of making the cutoff for a particular frame and hurt smoothness.

Fixed by https://chromium-review.googlesource.com/c/chromium/src/+/1176879