This is a tracker bug for work related to automating finding of tasks with non-ideal TaskPriorities.

Identified potential gains should be marked as blocking this bug.

Step #1 is to add browser tests that ensure we can startup+navigate without BACKGROUND tasks (issue 831835).

Further ideas :
 - Fuzz random USER_BLOCKING tasks to run as BACKGROUND, if we can still startup+navigate then that task shouldn't be USER_BLOCKING? (might need to define a few more user blocking actions that must be achievable without this task for it to be flagged, e.g. input-based like scroll).

 - Automate searching through Slow Reports traces for janky tasks on the critical paths and move them to TaskScheduler.