Before this CL, the caged heap was lazily initialized upon the first
call of HeapBase ctor. CagedHeap keeps a pointer to PageAllocator which
was provided from cppgc::Platform through the HeapBase ctor. This was
not generally safe: the platform is not enforced to be singleton. If it
happens to die first, then CagedHeap will have a stale pointer. The CL
fixes it simply by moving caged-heap initialization to
cppgc::InitializeProcess(), which already requires a constantly living
PageAllocator.

Bug: chromium:1338030
Change-Id: Ifb70a2db233ef36a99c919db09bed9ff9f3708ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732107
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81422}