Now that ReadOnlyRoots is used everywhere to access the read-only roots,
this makes the Heap accessors for such roots private.

It also adds tests that the roots reachable from ReadOnlyRoots are all
in RO_SPACE as well as tests that the roots still publicly accessible
from Heap are not in RO_SPACE. There's a white list in the file for
the few roots where the root pointer itself can change. (For instance
materialized_objects points to empty_fixed_array to start with before
before later pointing to a mutable array).

Also fixes up new use of heap->empty_fixed_array() in elements.cc added
since I cleaned it up.

Bug: v8:7786
Change-Id: I9ac7985c9f85910b5b22d2f9f559dfd04d43ed44
Reviewed-on: https://chromium-review.googlesource.com/1126252Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54240}