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Currently in the DeviceOrientation and DeviceMotion unit tests and
browser tests, each fake sensor creates a new shared memory handle.
While in the production code, only one shared memory handle is created
and all sensors share it, and write data to that shared memory using
different offset. This CL updates the FakeSensor and FakeSensorProvider
to match the behavior of production code.

Bug: 824591
Change-Id: I545d73b0d864a3b7093a387d007cab26fb912d91
Reviewed-on: https://chromium-review.googlesource.com/978856
Commit-Queue: Jun Cai <juncai@chromium.org>
Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547848}