Since we now add the additional required capacity when assessing
StringTable shrinking during LookupKey-caused string insertion, we no
longer have to add a slack capacity. This more closely matches previous
behaviour, which didn't consider the additional required capacity but
did add slack (which, because of rounding to the next power of two,
amounts to the same thing).

Bug: chromium:1108258
Change-Id: I81f513fdc800a2caab76e030c0c5e767cd1360d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2324253
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69127}