HeapObjectHeader::GetSize returns object size including
HeapObjectHeader. This is find for our internal usages since we want to
know the actual size consumed by the object.
ObjectSizeTrait should report object size for external users for which
the HeapObjectHeader should be invisible. The sizes returned by the
trait should exclude the header.

Bug: chromium:1056170
Change-Id: I75111b9a3d0d27d0a39df4abdacaa84f1cee3a5a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2692689
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72723}