Rather than having an optional script id during ParseInfo creation (which
is either selected lazily on script creation, or eagerly if based on an
existing Script), always eagerly get either the desired script id (either
from the Script or Isolate::GetNextScriptId()).

This has the side-effect that we will currently no longer need to get the
script id on background threads, but I'm not reverting the thread-safety
of Isolate::GetNextScriptId in case it's needed again in the future.

Bug: v8:10314
Change-Id: I8f2dd962d3652b1a84a5d704a099e57a1679aba5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2096616
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66649}