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.

Bug: chromium:818642
Change-Id: If86ff1b9845e8dd3b015b4e554d0033328b145bf
Reviewed-on: https://chromium-review.googlesource.com/1127046
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54263}