Make Hashtable Shapes return Map Handles (from the read-only roots)
instead of the root index of the Map, so that they can be used off
the main thread.

Bug: chromium:1011762
Change-Id: I4c0a8518dc1c6d490b5c04da05b5319081a6fae5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2083298
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66569}