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This CL is part of servicification work for //device/serial, it:
 - creates a new mojo interface SerialDeviceEnumerator and exposes it
   from Device Service, the interface implementation wraps
   a device::SerialDeviceEnumerator to do actual work.
 - lets serial extension API impls consume the above interface instead
   of directly calling functions of device::SerialDeviceEnumerator.

BUG=749514
TEST=browser_tests SerialApiTest.*

Change-Id: Ib768bb25032fcc4a0899780a2f85467f128a6800
Reviewed-on: https://chromium-review.googlesource.com/586632Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#491942}