Until now, the in-object properties on JSObject have been invisible to
tools using the postmortem debugging library. With this change, those
tools will get enough information to show a flat list of property
values. This is still less powerful than the runtime printers, which can
show the corresponding key for each value, but it's a big step up from
manually inspecting memory.

This change basically requires a reimplementation of
Map::GetInObjectProperties for postmortem debugging. I'm not
enthusiastic about duplicating this logic, but it's pretty small and I
don't see any good alternatives.

As a drive-by cleanup, I moved some inline string literals into a batch
of constexpr char arrays.

Bug: v8:9376
Change-Id: Ia24c05f6e823086babaa07882d0d320ab9a225db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1930174Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#65183}