Ulan's CL implemented CollectCustomSpaceStatisticsAtLastGC in CppHeap.
All our other methods just delegate to internal::CppHeap. This CL moves
CollectCustomSpaceStatisticsAtLastGC as well to align it with the rest
of our methods.

Bug: chromium:1056170
Change-Id: I4b92d1779247fe26abcb8c6754f2abbc0d80adbd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2882802
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74469}