There's no point in maintaining a separate counter for the size of a
`std::list`. Also changing the type to `size_t` consistently.

Bug: chromium:1257637
Change-Id: I4f938b9888bb09cd1223ae6b6ae1db0fa1181096
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3220332
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77376}