Previously, the rendering tasks (i.e. BeginMainFrame) were always run at default priority on the main thread. This means that rendering could be starved by other normal priority work, for example tons of postMessages (crbug.com/943621). This feature is an internal scheduling change which prevents this from happening by periodically increasing the priority of rendering if we have not rendered in a while. It targets a lower frame-rate of 10 fps.
Explainers: N/A, this is an internal change.