I set higher-than-necessary Nursery::SubChunkStep constants to try to prevent frequent changes in nursery size, but this isn't necessary and setting to the arena size is simpler.

I initially set this to 64K thinking that it'd prevent too much changing of
the nursery size and therefore (once implemented) too many calls to
madvise. But that's not true, because the nursery only changes size when a
tenure rate threshold is hit, and once it does it always changes size. So
we can reduce this to 4K for simplicity and alignment with other constants.

https://hg.mozilla.org/mozilla-central/rev/705b9ae4d728