Move some of PreParsedScopeData's santity checks, such as the magic
value separating skippable function data from scope data, to be debug
only, to save memory.

Start position of inner skippable functions is still kept, because it's
too good at catching bugs, but we may want to remove it in the future
as well.

Relanding unchanged after the (unrelated) flake it exposed is fixed in:
https://chromium-review.googlesource.com/1131503

Bug: chromium:818642
Change-Id: Id1d9fe757875cd05ea9a92b41e7256c3ee86fc8e
Reviewed-on: https://chromium-review.googlesource.com/1131505
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54380}