In future the RO_SPACE root accessors in Heap will become private, so
instead convert them all to use ReadOnlyRoots.

Bug: v8:7786
Change-Id: I6a7fb375411e379681637469bb3fb360635e6bbe
Reviewed-on: https://chromium-review.googlesource.com/1126111Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54214}