This is a reland of f25cb50a

Removed the problematic tests.
The problem with the test was that we try to pop from an empty segment.
GCC flags that as accessing beyond the array (i.e. index is uint16_t
equivalent of -1). Preceding the actual pop is a DCHECK that asserts
the segment isn't empty. In practice, since we have the DCHECK and
access to the segment is always via a Local, this shouldn't be a
problem.
Unfortunately, GCC flags the access regardless. The DCHECK goes through
a function pointer so GCC cannot determine that in our unittest the
DCHECK would crash if index is 0 and the access would not happen (The
indirection was added to allow for test DCHECK handlers that don't
crash, so we can't mark the function pointer as noreturn).

Drive-by: Segment::Pop and Segment::Push rely on the their Local
counterparts checking of emptiness/fullness, so we should always
access segments via Locals. Making the Segment ctor private.

Original change's description:
> Reland "cppgc, heap: Don't eagerly allocate worklist segments"
>
> This is a reland of c99147c6
>
> Original change's description:
> > cppgc, heap: Don't eagerly allocate worklist segments
> >
> > Bug: chromium:1056170
> > Change-Id: I75a6b5f52bfe8dd71abc086e5d1e060759ad7fc0
> > Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2391254
> > Commit-Queue: Omer Katz <omerkatz@chromium.org>
> > Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> > Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#69778}
>
> Bug: chromium:1056170
> Change-Id: I4633da065976a6b2710d2f23b946fd2af0e65c83
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2401425
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Omer Katz <omerkatz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#69806}

Bug: chromium:1056170
Change-Id: I7a122d1a2d20cd4e7c824d249975b4d3df30e03e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2403251Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69829}