Currently BACKGROUND tasks are executing in a background worker pool. Unfortunately not all platforms support proper background thread priority [1].

On those platforms TaskScheduler needs to more aggressively throttle BACKGROUND tasks by moving everything into one pool and having a quota on how many workers BACKGROUND tasks can use even when idle (to avoid having a starved pool when foreground work comes in).

Step #1 is to resolve issue 831835 to ensure no BACKGROUND work is required to startup + navigate.
In parallel with step #1 we can strip the background pool on platforms that don't support background threads.

Then:
 - Add ScopedNoBackgroundTasks() for max responsiveness situations like startup or incoming-scroll.
 - Enable aggressive backgrounding.

[1]
ThreadPriority SchedulerWorker::GetDesiredThreadPriority() const {
  // All threads have a NORMAL priority when Lock doesn't handle multiple thread
  // priorities.
  if (!Lock::HandlesMultipleThreadPriorities())
    return ThreadPriority::NORMAL;
  ...
}

As of today Lock::HandlesMultipleThreadPriorities() is only true on Mac/Windows (i.e. not Android/Linux/etc.).