This is a clean-up to completely remove the Heap accessors for roots
accessible via ReadOnlyRoots. Where previously ReadOnlyRoots called the
Heap accessor, now it accesses the roots array directly using the root
index (since ReadOnlyRoots is a friend of Heap).

Also clean up several cases where private Heap accessors were still
being used by Heap and its friends.

Bug: v8:7786
Change-Id: Iaca2e17b22822b30d395dec6250a4d5ae496c983
Reviewed-on: https://chromium-review.googlesource.com/1127172
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54297}