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Add support for External Data referenced by Device Policy.
Use the new support to implement the DeviceNativePrinters policy.
Add browser test verifying external data cache is cleaned up after policy was changed or removed.

Bug: 807068
Change-Id: Ib98778a438e929a78214d1d9ab066a41a6bdcf09
Reviewed-on: https://chromium-review.googlesource.com/c/1335592
Commit-Queue: Nikita Podguzov <nikitapodguzov@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610596}