Split the capacity calculation logic and the allocation logic for
StringTable resizing (both shrinking and enlarging).

This allows us to:

  a) Precalculate the new size, without potentially first shrinking then
     resizing again,
  b) Make the final size calculation more explicit,
  b) In the future, maybe allow the allocation to happen outside a lock,
     and the rehashing to be inside it, for a concurrent string table.

Change-Id: Ie41773520f2b20d9d244c151a153ed784f17283e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2307218
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68958}