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

Bug: v8:7786
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I2f2c031c03d56d360ef940fc925e0583e6ae31dc
Reviewed-on: https://chromium-review.googlesource.com/1125720Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54202}