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This is a reland of b5244048

The conflicting import has been removed.

Original change's description:
> [iOS][Credential-Provider] Move account validator to common 1/3
>
> Moves files to common to be used in Chrome.
>
> Bug: 1066803
> Change-Id: I6f3ec5e7916a94cdf37d20be3138e8e99ebb2257
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2157215
> Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Reviewed-by: David Jean <djean@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#761452}

Bug: 1066803
Change-Id: Ie8658dfbc3323708330844e82e12b5fe0c42606c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2161033Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: David Jean <djean@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761979}