Split platform into a process-global initialization part and per-heap
platform objects.

These platform objects still contain allocators and executors. With
per-heap platforms GetForegroundTaskRunner() returns by definition the
correct runner.

In future, when initialized throuhg V8, an adapter can be used to
translate between the different platforms, avoiding the needed for V8
embedders to provide additional information.

Bug: chromium:1056170
Change-Id: I11bdd15e945687cfbdf38cae4137facb02559e0a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2218030
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68059}