Having O(1) access to the length of the free lists on a given Page
will be useful to compute fragmentation and to implement efficient
heuristics to select evacuation candidates.

Bug: v8:9329
Change-Id: I92c7fcc38c89dcb18fef4ce7cc9ebf0b83d03dc5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1664065Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@google.com>
Cr-Commit-Position: refs/heads/master@{#62251}