This CL adds the new AllocationTypes kSharedOld and kSharedMap for
the allocation of objects in the shared heap. An Isolate can be converted
into a shared isolate with Isolate::UseAsSharedIsolate(). Client
isolates can then be attached to the shared isolate.

Bug: v8:11708
Change-Id: Ie2ed30b83b404e01e6f6079c5a4bdaa9302e8004
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2850709Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74252}