Make PreParsedScopeData a new array-like instance type, which holds its
child data inline, rather than indirecting to a FixedArray. Should save
one map word per PreParsedScopeData.

Reland of https://chromium-review.googlesource.com/1127055, adding
padding clearing to keep MSAN happy.

TBR=jarin@chromium.org,marja@chromium.org,ulan@chromium.org

Bug: chromium:818642
Change-Id: I536db452047959e75d5116ddded4f511d05a04d9
Reviewed-on: https://chromium-review.googlesource.com/1131512Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54374}