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This is intended to be used as an indication to the UI that the user
should take action to improve recoverability of the sync-ed data.

This patch introduces basic plumbing in TrustedVaultClient with TODOs
to address platform-specific implementations in later patches.
Meanwhile, StandaloneTrustedVaultClient is extended with test-only
APIs for integration tests.

Change-Id: Ic255e502d9e33b145272903dc2b7c3a7ec2a1cd8
Bug: 1081649
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2329751Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793231}