*Scan: Avoid metadata allocations in SuperPageSnasphot

std::vectors that are currently used inside SuperPageSnasphot cause
committed memory of the metadata partition to grow by 2x. The CL avoids
it by using on-stack arrays. ScanArea::begin/end pointers are packed so
that the array doesn't take up too much space (<1K on 64bit) to minimize
the risk of stack overflow.

Bug: 11297512
Change-Id: Ie5dd1ecc14f5d2cb76233729a1521fd9017ec582
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3015344


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900960}