Reland "Add `CheckedRandomAccess[Const]Iterator`."

This reverts commit 5f80b9a84001d27481ac978d2a545132f2e75e61.

Reason for revert: Revert was wrong; it was OK the first time.

Original change's description:
> Revert "Add `CheckedRandomAccess[Const]Iterator`."
> 
> This reverts commit 383049f284c02d11f4026b365c8a4d902185dbb8.
> 
> Reason for revert: Landed wrong patchset!
> 
> Original change's description:
> > 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}
> 
> TBR=ajwong@chromium.org,palmer@chromium.org,dcheng@chromium.org,jyasskin@chromium.org,mmenke@chromium.org,tsepez@chromium.org,nharper@chromium.org
> 
> Change-Id: Icf99f8409054d853a15b8d27cdb232ff727f8a42
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 817982
> Cq-Include-Trybots: luci.chromium.try:linux_mojo;master.tryserver.chromium.linux:linux_mojo
> Reviewed-on: https://chromium-review.googlesource.com/1211743
> Reviewed-by: Chris Palmer <palmer@chromium.org>
> Commit-Queue: Chris Palmer <palmer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#589245}

TBR=ajwong@chromium.org,palmer@chromium.org,dcheng@chromium.org,jyasskin@chromium.org,mmenke@chromium.org,tsepez@chromium.org,nharper@chromium.org

Change-Id: Ieba0af822a2287583cf37a109538f6ceeb8e4f98
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 817982
Cq-Include-Trybots: luci.chromium.try:linux_mojo;master.tryserver.chromium.linux:linux_mojo
Reviewed-on: https://chromium-review.googlesource.com/1211902
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589278}
7 files changed