When RO_SPACE is to be shared, this constructs SharedReadOnlySpace
object (via ReadOnlySpace::DetachPagesAndAddToArtifacts) that contains
the shared artifacts and the original ReadOnlySpace is destroyed. This
is mostly a conceptual change and SharedReadOnlySpace behaves
identically to ReadOnlySpace (and subclasses it).

Also adds ReadOnlyArtifacts that contains the shared artifacts and which
is stored as a std::weak_ptr in a global so it can be destroyed when all
std::shared_ptrs to it are destroyed. Since this allows the ReadOnlyHeap
to be reconstructed when all Isolates are destroyed,
ReadOnlyHeap::ClearSharedHeapForTest is removed along with all uses
since that is now done automatically.

The ReadOnlyArtifacts class now owns all the shared artifacts and is
responsible for deleting them on exit (mostly via unique_ptr).

Bug: v8:10454
Change-Id: I2fe7110a4ab9cf8719dd198bafc1d083bee641b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2154204
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67440}