... by removing entries corresponding to read only roots (which are
immortal immovable by definition) and using READ_ONLY_ROOT_LIST explicitly.

This CL also renames the list to MUTABLE_IMMORTAL_IMMOVABLE_ROOT_LIST and
moves Heap::RootIsImmortalImmovable() to RootsTable::IsImmortalImmovable().

Bug: v8:8238
Change-Id: I3e44a06d7a816955bc3471e788e883fb053b03d9
Reviewed-on: https://chromium-review.googlesource.com/c/1269035Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56466}