... and remove Heap::RootCanBeWrittenAfterInitialization() and
Heap::RootCanBeTreatedAsConstant() in favour of RootsTable::IsImmortalImmovable().

Bug: v8:8238
Change-Id: I804d06136de9584b8c4940fd8ab9d18fb3ef7980
Reviewed-on: https://chromium-review.googlesource.com/c/1270837
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56500}