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This adds basic support for persisting encryption keys on disk using a
dedicated file (intentionally outside sync's directory), containing an
encrypted protocol buffer as introduced in this very patch.

Bug: 1012660
Change-Id: Ie9fedae86799f412703914dbfec53e3d0b85a2c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865324
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707986}