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Make a keyed service from the platform_keys.h file and add
a mock class for it to allow unit testing of a code that
depends on that code.

Bug: 1045895
Change-Id: I5008805ac7aaf85af65177d67b45a9411c1726a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078614
Commit-Queue: Michael Ershov <miersh@google.com>
Reviewed-by: Edman Anjos <edman@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748197}