Introduce an IsolateRoot class, which encapsulates the root address
needed for pointer decompression. This class is implicitly constructible
from both Isolate* and LocalIsolate*, allowing us to avoid templating
methods that can take both, or awkwardly creating a `const Isolate*`
from a `LocalIsolate*` just for getters.

Change-Id: I6d4b9492409fc7d5b375162e381192cb48c8ba01
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2440605
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70365}