Add range checking to `base::span`.

Bug: 817982
Change-Id: I823b9203db0f549c19f9de7f5ac742ba358b8858
Reviewed-on: https://chromium-review.googlesource.com/952187
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542337}
2 files changed