The new space is unused in the snapshot, as we convert all new objects
to old space objects when serializing. This means we can get rid of
the snapshot new space entirely, and as a result get rid of the write
barrier checks.

This also rejiggles the order of the general spaces enum so that the new
spaces are at the end, and can be truncated off for the SnapshotSpace
enum.

As a drive by, fix a bug in an unrelated test-api test which this patch
exposed.

Change-Id: If67ff8be5bf03104a3ffae7df707c22460bba3a1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2390762
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69761}