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It's used for poking at InputDeviceClient. As Chrome/Ash use
DeviceDataManager in classic mode InputDeviceClientTestApi handles
both. That is, if DeviceDataManager exists it pokes at it, otherwise
InputDeviceClient.

BUG=776514
TEST=test only changes

Change-Id: I4e46dbe74dbce4a37d2b4e054b8fec88d4b5a048
Reviewed-on: https://chromium-review.googlesource.com/829713Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524447}