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This change creates a way for tests to control the behavior of
the fake OpenVR, and allows devices to send information up to
tests.

BUG=801034

Change-Id: Ie0fe035ada9553114c054717f52788d375d5c62f
Reviewed-on: https://chromium-review.googlesource.com/1100157
Commit-Queue: Bill Orr <billorr@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567725}