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This is a reland of 2d2185d8

The fix is to remove the ifdef check. I incorrectly assumed that DCHECK
would strip the code on release configurations.

Original change's description:
> [iOS][Credential-Provider] Observe changes in the password store
>
> Updates the credential store with changes from the password store.
> Renames SyncAllCredentials to RequestSyncAllCredentials to make clearer
> the async nature of it.
> Creates entry functions for adding, updating, and removing passwords
> which will be used to also keep up to date the OS metadata in a follow
> up CL.
> Injects the credential store for easier testing.
>
> Bug: 1066803
> Change-Id: Id79c3b276517c7bb223b3988db4d1a8c25e00025
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152814
> Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
> Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#760483}

TBR=vasilii@chromium.org

Bug: 1066803
Change-Id: Ie50f790c6cc8715c13017fc930327019f137fb3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2157045Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760564}