This was missing from Ulan's implementation for
CollectCustomSpaceStatisticsAtLastGC.

Bug: chromium:1056170, chromium:1181269
Change-Id: I72354e4f04873095eac5cb39ed188ed83de0bd26
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2880219Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74483}