Disable the checksum comparison for the read-only snapshot when
--stress-snapshot is used, since it's possible that it would be
corrupted. This corruption is not important as the purpose of
stress-snapshot is not to produce a useable snapshot, but to test that
the serialization/deserialization does not fail for any given objects.

Since the --stress-snapshot flag's value is now used outside of d8,
this also moves it to flag-definitions.h.

Cq-Include-Trybots: luci.v8.try:v8_linux64_gc_stress_custom_snapshot_dbg_ng
Bug: v8:11750
Change-Id: Iedcf1cfb5afa5f16ac19a76820b62b5b93948f2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2882810
Commit-Queue: Dan Elphick <delphick@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74480}