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: I631fa1e44d2b9998a5567c96c6977955ac86d5a1
Reviewed-on: https://chromium-review.googlesource.com/1125931Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54215}