Model cppgc::InitializeProcess()/cppgc::ShutdownProcess() similar to
V8's InitializePlatform()/ShutdownPlatform() in that we allow the pair
to be called multiple times.

GCInfoTable will not be freed on ShutdownProcess though as the current
global design uses static indices to retrieve per-type metadata.

Drive-by: Remove stale ShutdownProcess() call.

Change-Id: Ia9b50325a964e85a72f3ef218e72bc386b69be51
Bug: chromium:1176416, chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2685171Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72630}