This will help reducing the time needed to reach a Safepoint() on the
main thread. During startup main_thread_local_isolate() is not
initialized when Heap::AllocateRaw() is invoked. Solve this by
only running Safepoint() after deserialization is completed.

Bug: v8:10315
Change-Id: I281fdbe5cebcd7946d687f56676c0d563792fde5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2835714
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74181}