Checked Span: Enable tightened checking

This enables tightened bounds-checking. Checked Span now refers to the
hidden, originally requested size provided by the caller, and will crash
if a `base::span` any larger than that is requested. (Exceptions apply,
specific to the implementation of PartitionAlloc.)

Fixed: 534387022
Change-Id: Id74f51b31ba2e59c6a5b7889305f54b0476c1340
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8491851
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1709778}
NOKEYCHECK=True
GitOrigin-RevId: a27e1a2c28881022a9eb7a7d2f0d1e7f90ecafc4