Embedders may use cppgc (or v8::CppHeap) earlier than V8's Isolate and
platform are initialized. Require explicit initialization of cppgc to
avoid recurring init calls with potentially conflicting parameters.

Bug: chromium:1056170
Change-Id: I613452954b322c9a5bf074eefd25107b4579958c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2682648Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72573}