So far, discarded size was maintained by the sweeper but not wired up
anywere.

Changes in this patch:
- Wire up resident size in heap statistics collection.
- Fix bugs in reporting committed and resident size.
- Sweeper test: Enforce some internal details. The details should not
  not be checked broadly but be kept as a detail to the sweeper
  itself.
- Stats collection: Test that committed and resident set size are
  reported and differ after discarding GCs.

Bug: chromium:1056170
Change-Id: Icf8871c7ea3b28253233485c736b2ca4816fd6f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3020971Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75684}