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This interface abstract the synchronisation of device accounts with
the IdentityManager. The goal is to promote the "legacy" methods
LegacyReloadAccountsFromSystem() and LegacyAddAccountFromSystem()
to supported method but with a nice abstraction that explain why
they are needed (i.e. access to accounts stored on some device
abstraction).

Add implementation (restricted to iOS for the moment, but could be
enabled on Android if needed) that just invoke the same code as the
old methods.

Convert iOS code to use the new interface and remove the old ones
(except LegacyAddAccountFromSystem which is now marked as Android
only).

See design at:
https://docs.google.com/document/d/1_NknywZB7UCQdCiCxEPQU85lXouid_aMxdzujEJSiEM/preview

Bug: 930094, 957887
Change-Id: I09fb20ea72c4cf584079e975276367c33392f416
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1690965
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676391}