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This CL adds high-level trusted vault key rotation logic. Once vault
keys are marked as stale, following FetchKeys() call will trigger
attempt to download fresh keys from the vault server. This makes
StandaloneTrustedVaultBackend::FetchKeys() asynchronous, but result
is populated immediately if preconditions aren't met (e.g. device isn't
registered, device somehow misses old vault keys or current account
isn't syncing one).

StandaloneTrustedVaultBackend allows only one ongoing request to the
vault server and only one ongoing fetch keys attempt at the time
(following TrustedVaultClient::FetchKeys() documentation). In case of
abandoning download keys request, ongoing fetch keys fulfilled
immediately.

Bug: 1102340
Change-Id: I2413176f03b07e67a72315a549add16f5e328678
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340977
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799624}