Process global accessors for total physical size and used size are
needed for blink. These are implemented via an allocation observer that
atomically updates static counters.

The public api only provides getters for the counters. An internal class
is in charge of updating the counters as needed. A similar split is also
applied to IsAnyIncrementalOrConcurrentMarking().

Drive-by: ProcessHeap is merged into cppgc::internal::WriteBarrier.

Bug: chromium:1056170
Change-Id: Iaedebd1ac9d49238ce6bdd52ffa5d1ef4d28203d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2695394
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72832}