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The abstraction allows injecting platform-specific logic about
retrieving locally known encryption keys, as well as the optional
ability to store them (independently of sign-out state or sync engine
initialization state).

The API is designed to be asynchronous in order to support IO or IPCs.

SyncServiceCrypto is extended with an intermediate state where keys
are being fetched, before actually prompting the user for action.

Bug: 1012659, 1012660
Change-Id: Ia8daa317a4fbc1cbb50be7e2c0db40a1705f8b66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849381
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704644}