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: I315e63a30ca6f3077f18eb1a5004daefba9bc673
Reviewed-on: https://chromium-review.googlesource.com/1125929
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54223}