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Add a caching factory for Chrome OS |AccountManager|. Cache is keyed on
the basis of Profile path (|Profile::GetPath|).

AccountManager cannot be simply a part of |g_browser_process| because of
Chrome OS Multi Sign In. Accounts participating in Multi Sign In should
see separate instances of |AccountManager|.
Once Multi Sign In has been replaced by Multi Profile,
|AccountManagerFactory| can be removed and |AccountManager| can be made
a part of |g_browser_process|.

Bug: 820046
Change-Id: I8c2f2b2b5d8d61c5a818ab711197840bd605567b
Reviewed-on: https://chromium-review.googlesource.com/1010542Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551176}