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Add a Sync integration test which exercises custom passphrase flows, including
the handling and proper functioning of the newly added key derivation method
(scrypt).

Add an integration test helper file for encryption-related tasks and modify
FakeServer and LoopbackServer to allow easier modification of persistent
entities such as Nigori. Add passphrase-related functionality to
ProfileSyncServiceHarness to give tests better control over how and when
passphrase-based encryption is enabled.

The test uses a gray-box approach, where it tests the client at the
ProfileSyncService granularity, but injects and inspects entities on the (fake)
server to ensure that encryption is performed properly. This is necessary
because, when it comes to encryption, we are not interested merely in that it
does not hinder existing functionality (e.g. two clients are syncing data
properly in the presence of a custom passphrase), but also that it provides the
expected security to the user. For this reason, we use our knowledge of the
encryption architecture internals to ensure that the data committed to the
server is encrypted in the expected way.

Bug: 894148
Change-Id: I728f7f18cc0db7b1da50f747a87a640877d0b023
Reviewed-on: https://chromium-review.googlesource.com/c/1274205
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@{#598912}