HeapBase::CollectStatistics returns a HeapStatistics struct that can be
used by blink to populate a memory dump.

Bug: chromium:1056170
Change-Id: Ic147a02ba6b4aa77bf92cfca067da70b7e1af55b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2689181
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72660}