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This is a reland of 48c7a842

Original change's description:
> [sync::test] Add two client integration test for custom passphrase
> 
> Add a Sync integration test which exercises custom passphrase flows in
> an end-to-end manner with two clients. The test ensures that two clients
> can sync data properly in the presence of custom passphrase-based
> encryption and different key derivation methods.
> 
> Refactor SetScryptFeatureState() from the single-client test to a scoped
> feature toggler that resides in encryption_helper, because the
> functionality now needs to also be used from the two-client test.
> 
> Bug: 894148
> Change-Id: Ia211b221f89a99092d3b19690ecae6bb4fa77d02
> Reviewed-on: https://chromium-review.googlesource.com/c/1277648
> Commit-Queue: David Davidović <davidovic@google.com>
> Reviewed-by: vitaliii <vitaliii@chromium.org>
> Reviewed-by: Marc Treib <treib@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#599929}

Bug: 894148
Change-Id: I425db59e1427f2c3ebf1cb9cd8b2ee440fd31ce1
Reviewed-on: https://chromium-review.googlesource.com/c/1283012Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: David Davidović <davidovic@google.com>
Cr-Commit-Position: refs/heads/master@{#599945}