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This is a reland of 33cfbf08

Changes (fixes):
- Add CHROMEOS_EXPORT to AccountManager
- Add failing linux-chromeos-dbg trybot

Original change's description:
> Add Chrome OS Account Manager
>
>   - Loads Login Scoped Tokens (LSTs) from disk to memory
>   - Persists updates to LSTs to disk
>
> Design doc is linked in the bug id.
> This CL creates the core of Account Manager and its ability to load
> and persist its state.
> Follow up CLs will:
> - Link it with OAuth2TokenService via an OAuth2TokenServiceDelegate.
> - Add UI components to modify Account Manager's state.
>
> Bug: 820046
> Change-Id: I1b43e11cfc7ed3592daf4a79ff664d377644f6a8
> Test: chromeos_unittests --gtest_filter="*AccountManager*"
> Reviewed-on: https://chromium-review.googlesource.com/955523
> Commit-Queue: Kush Sinha <sinhak@chromium.org>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#543944}

Bug: 820046
Cq-Include-Trybots: master.tryserver.chromium.chromiumos:linux-chromeos-dbg
Test: chromeos_unittests --gtest_filter="*AccountManager*"
Change-Id: I68b906ef1288153c26f70735dfcb71790cde520d
Reviewed-on: https://chromium-review.googlesource.com/968486
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544115}