Add `CheckedRandomAccess[Const]Iterator`.

And add a first call site.

Bug: 817982
Cq-Include-Trybots: luci.chromium.try:linux_mojo;master.tryserver.chromium.linux:linux_mojo
Change-Id: I2db298494e4f562ffac1f68b87c7a617e592f196
Reviewed-on: https://chromium-review.googlesource.com/963726
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Jeffrey Yasskin <jyasskin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589244}
7 files changed