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This commit adds fake classes: FakeSensorService and FakeSensorDevice in
chromeos/components/sensors to add unit tests of sensor clients in
Chromium, such as ash/accelerometer and Generic Sensor.

BUG=chromium:1006141
TEST=unit tests of ash/accelerometer

Change-Id: If4cab4907a3db0e84214ec274ce9dab0a7f2cc44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449311
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815053}