The call sequence for snapshot creation is quite involved. Details
should not be exposed outside src/snapshot. This CL pulls more details
of snapshot creation into Snapshot::Create, which is intended to be the
single chokepoint for API, tests, and other internal use.

Bug: v8:10416,chromium:1043058
Change-Id: I610293b5f70fec12e5513e5f803b5dd9118fd3b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2150589
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67384}