To get the Isolate from a HeapObject, rather than masking off the
MemoryChunk and then loading the heap from the MemoryChunk (which won't
work when RO_SPACE is shared between Isolates), get the Isolate by
masking off the bottom 32 bits and apply the Isolate bias.

Also fixes up a stale comment and makes several methods in RootsTable
and Isolate const to support this change.

Bug: v8:10454
Change-Id: I5f8eb873d8486b699460223dbe3454a5dcf1854f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2280088
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68671}