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

Splits HEAP_CONSTANT_LIST into HEAP_MUTABLE_IMMOVABLE_OBJECT_LIST and
HEAP_IMMUTABLE_IMMOVABLE_OBJECT_LIST. The CSA accessors then use the
appropiate ReadOnlyRoots or Heap accessor to get hold of the object.

Bug: v8:7786
Change-Id: I3bdf868b5e224f8794c85307e1b4569480cebd1c
Reviewed-on: https://chromium-review.googlesource.com/1125725Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54199}