GitLab

This is a follow-up to crrev.com/c/2329751 to support notifying
observers, along with an API to resolve the recoverability errors.

The actual TrustedVaultClient implementations are left unimplemented,
except for StandaloneTrustedVaultClient where minimal logic is
introduced to support semi-realistic integration tests.

Change-Id: I63acc6196fee59d24edbd1d3816f19ef8e15930a
Bug: 1081649
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2448491
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#813716}