and introduce RootsTable - a V8 heap roots storage.

So, the renaming part looks like this:
  Heap::RootListIndex -> RootIndex
  Heap::kBlahBlahRootIndex -> RootIndex::kBlahBlah

Bug: v8:8015, v8:8182
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I38e1f3e3f6813ef35e37b0bed35e9ae14a62134f
Reviewed-on: https://chromium-review.googlesource.com/1234613Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56067}