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In this patch, GaiaScreenHandler adds support to deal with the received
sync trusted vault keys.

This information is wrapped in a new class (SyncTrustedVaultKeys) and
propagated via UserContext to UserSessionManager, where during profile
preparation is propagated to SyncService and consumed.

Browser tests are introduced to exercise the whole integration, from
the server (FakeGaia) to the final storage (TrustedVaultClient).

Change-Id: I3cdc16086b2eec2b2a603255ed644c7b1f53192e
Bug: 1081651
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590035
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846285}