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Create() returns std::unique_ptr<> instead of a raw pointer.
UpdateEngineClient will be addressed later as it has 3 implementations
(i.e. real, stub, and fake).

BUG=952745
TEST=build

Change-Id: I5153d7c1aefb244c24b7bc335d3678f92729d6ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574996
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652815}