Add |token| field in SerialPortInfo and use it in GetPort().

A new field |token| has been added in SerialPortInfo and used in
GetPort() as unique identify of a serial port without referring
to its OS identifier.

BUG=908833

Change-Id: I5d46f5601a5453ae2f8f937713dd87b12a3849f2
Reviewed-on: https://chromium-review.googlesource.com/c/1375456
Commit-Queue: Donna Wu <donna.wu@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620271}
35 files changed