PagedSpace::Available() is mostly used for counters. One may expect
that it'd be constant time or bearly noticeable performance-wise, but
its cost is linear in the number of freelists and number of pages in
the freelists. Overall, d8 --prof showed that it has a important
runtime cost, and prevents freelists from scaling.

This CL makes this counter constant-time, and should improve
performances, even using with our current FreeList strategy
(FreeListLegacy).

Bug: v8:9329
Bug: v8:9093
Change-Id: I7682c5debc78498fe46e8dbce70b2fbd540b0fd0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1746473
Commit-Queue: Darius Mercadier <dmercadier@google.com>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63197}