Revert of [Autofill Clean up] Revert changes introduced by Autocheckout to autofill (patchset #1 id:1 of https://codereview.chromium.org/609733003/)

Reason for revert:
these changes are still necessary for requestAutocomplete

Original issue's description:
> [Autofill Clean up] Revert changes introduced by Autocheckout to autofill
> disabled and readonly fields. Autocheckout is deprecated.
>
> R=isherman@chromium.org
> BUG=231160
>
> Committed: https://crrev.com/d9b040ba4ff2ff78f183816b9d0e34c5f450160c
> Cr-Commit-Position: refs/heads/master@{#297015}

TBR=isherman@chromium.org,ziran.sun@samsung.com
NOTREECHECKS=true
NOTRY=true
BUG=231160

Review URL: https://codereview.chromium.org/648243003

Cr-Commit-Position: refs/heads/master@{#299331}
3 files changed